You're part of the group
You say in your question that "the GM has to ask the players 3 questions". That's not true, though. The End of Session move says the following: (emphasis mine)
Then answer these three questions as a group
That means that you and the others all have basically equal say in answering the questions, though you as a GM can't mark xp (which is fine, you should be a fan of the characters and thus basically you should want them to become stronger). Thus, it depends entirely on your (whole) group for what you want to give out xp and for what you don't want to give it. That is a discussion you should have at your table.
Getting xp fast is not actually in the player's interests (that much)
You will probably see that the players will start to accrue xp slower as they go on and level up. They need more xp for each levelup, they roll a 6- less often (thus gaining less xp that way) and they might not actually want to earn xp that fast anyway. This is because of what happens when you reach level 11:
Once you’ve reached 10th level things change a little. When you have enough XP to go to 11th level instead you choose one of these:
- Retire to safety
- Take on an apprentice
- Change entirely to a new class
This means that they will have to basically start over from 0 (well, they get to keep their attribute scores when switching to a new class). And this is non-negotiable, at least if you are playing by the rules (which, in Dungeon World, you really should.)
If players want to keep their characters for longer, they need to level up more slowly. And they will come to see that. Or they might not care.
All of this is a good thing
At least kind of. Your players seem to want to get new abilities and levelups. That's a good thing! You will have much to explore. Where do these abilities come from? How do they work in your world? What does it mean for the inquisitor to be able to "smell guilt"? (Inquisitor is not a base class, I am using this example because it was a memorable one from my last 3 year long Dungeon World campaign that ended a few months ago). Still, there are two more things:
The questions are more restrictive than you realize
Let me go through your examples one by one:
"I found a bear-owl, so I found out bear-owls exist in this world (discover something)
That is a perfect answer to the question "Did we learn something new and important about the world?". This question should basically be answered with yes almost all the time, in my experience. Rarely does a session go by where you don't learn something new about the world. One of the most important things about Dungeon World is "play to find out what happens" after all. It has to be both new and important though. Simply finding out that grass is green in this world too is not enough. Finding out that grass is actually green in hell, after you heard all campaign that it would be black, burned and poisonous, might qualify though. That's for you and your group to decide together.
We scared it by blowing a horn (defeat weak but rare enemy)
The question does not say anything about rare (or weak) enemies though. The question is "Did we overcome a notable monster or enemy?". Was this Bear-Owl a notable foe? If yes, it does not matter if it was easy to get rid of in the end. Players come up with unexpected and creative solutions for stuff, in Dungeon World even more so than e.g. in DnD (at least in my experience). My group once fought a dragon, which was dangerous, but they were clever and lucky in approaching it, and then the fight was over after the cleric cast a spell to cut off its wing, duplicated it because of a good roll (thus cutting of both wings), the dragon crashing to the ground and the ranger shooting a few arrows in its eyes and throat. Those were basically two actions, and they were all it took. Did it feel awesome? Hell yeah. Did the group gain xp for that? You bet! We all agreed that they earned it.
That's how we found the treasure of music (treasure can be anything the characters value)
The question is "Did we loot a memorable treasure?". There was no "looting" involved, and it has to be memorable too. Looting a few coins from a bandits corpse? Probably not. Looting the legendary "Tooth of Time" from a dragon's hoard? Absolutely. Everything in between? You and your group have to decide. And talk about it. Over time, you will come to an understanding about what counts as "memorable" treasure for your group and what does not.
But my players come up with weird reasons (or reasons I disagree with)
That is a valid criticism. The question system is weird (compared to a more "normal" rpg like DnD I mean), and it's weird from both a player's and a GM's perspective. It takes some getting used to. And it takes some time to understand that its purpose is not simply to "get xp". The purpose (and the way it starts to work kind of by itself sooner or later, at least in my experience) is driving the players towards doing these things: Explore the world, Fight badass monsters and loot their treasure.
If you don't think that the answer to a question should be yes, say so. (But make sure to do it the other way around too, I often had to tell my players "I think that's a really important thing you found out about the world. Why don't you think it's important?"). And make sure to tell them that you don't say that just to "take away" the xp, but instead to make sure that the questions work as intended (see my last paragraph). And also: Make sure that this is actually why you dislike the answers your group gives. If it is because you think they are "cheating" or "overleveling": Both of these things do not exist in this context. It's not possible to cheat on these questions. Lie? Maybe. Cheat? No. And if you are afraid that they are overleveling or going to become overpowered:
There is no such thing as "overpowered" in Dungeon World
Well, technically there is, I guess. Characters can become strong. Like, really strong. The game wants them to be. The game wants them to have exciting adventures and to feel like badass heroes. It's literally the thing the game is good at. It's your job as a GM to make their lives exciting and fill them with adventure. And if you do that job correctly, they will not be any more or less overpowered than you (and the game) wants them to be.