This is a misreading of the rules.
Step 2: Add up Encounter XP
Total the XP values of every enemy creature in the encounter to get
the encounter’s XP value.
This is the actual XP the players will get (divided up by the number in the party) based on defeating this encounter. If you fight 10 ogres, it's ogre XP times 10. However, this is not the final number used to calculate the difficulty of the encounter, which comes next.
Thus, if you have an encounter with 4 monsters in it, multiply the
total XP value of the encounter by 2 for the purposes of determining
how difficult the encounter is. This doesn’t change the actual XP
award the adventurers receive for overcoming the monsters, just your
calculations of how difficult the encounter is.
The bolded is important. It means that this step is solely about determining the difficulty on the Encounter Difficulty XP per Character chart, not the XP the players will get.
So let's say 20 monsters who are 10 XP each are attacking the party. Together, the party will split 200 XP for beating it. But how difficult is the encounter? It's as difficulty as an 800 XP encounter (200 xp multiplied by 4 for 15+ enemies), but you're only getting 200 from it.
What it does
First off, it accounts for certain things that come up with multiple monsters:
- Economy of actions - more monsters grants more actions to the DM
- Map space - a lot of monsters allows the DM to control the movement on the board better
- Support multiplier - monsters who can buff or heal each other can get significant advantages in large numbers
- Increase number of attacks = increased odds of critical hits
So taking that idea of 20 monsters of 10 XP, they'd be a "medium challenge" for 4 PCs if you didn't take into account that multiplier - but with the multiplier, it is easily double the number for "Deadly". It helps the GM not overwhelm the party given the factors I listed.
At the same time it acknowledges the difficulty of mass numbers, it doesn't give further REWARD for mass number combats. You may still want to take out those 20 monsters - it's just going to be more to your benefit as players to take them apart in smaller units a bit at a time, rather than trying to rush them in one battle.
Add up all the monsters' XP. This is how much the party will get if they defeat the encounter.
Multiply that total by the Encounter XP Multipliers SOLELY FOR THE PURPOSE of checking how difficult the encounter will be compared to the party.