In addition to options only available to druids, like the Druidic language, and options only available to rangers, like the Advanced Ranger Trap feat, there is one ability that all hunters have whose various upgrade options key off of Druid levels-- the Animal Companion.
While it is true that hunters first count as a druid of their level for animal companion purposes, once they get Nature Training they instead count as a druid of their level and a ranger of their level (that is, a druid of their level*2-3) for animal companion purposes. At level 3, when a Hunter gains nature training, the ability breaks even, but at higher levels counting as both is better. Presumably, you only gain nature training at 3rd level because counting as both classes before that point would actually be bad for you in terms of animal companion strength, and that's very much a cornerstone of the Hunter class. This behavior may well be deliberate, as other hybrid classes typically gain their equivalent "I count as both" ability at level one, for example the Brawler's Martial Training.
So not only does the ability not do nothing, but it's actually a very important part of the Hunter class.
So what are the stats of the companion of a lvl 20 hunter? Do you have an explanation for why there is nothing on the class about how to handle this case?
Response to Objection 1:
There are a lot of abilities in Pathfinder, and indeed 3.x, that lead to undefined behavior at high levels due to things going off a chart. Sometimes the chart comes from clear rules and so extending it is simple, if weird. Sometimes there aren't and the behavior is truly undefined. Some examples include weapons that deal so much damage at Gargantuan size that increasing them to Colossal size moves the weapon off the damage by size chart, being told to advance a prestige class past the number of levels it possesses, and a monk of 16th level or higher donning a Monk's Robe. It should not be considered unusual that a bonus causes something to exceed unaugmented 20th level performance at high levels, since we can see that is fairly common, but rather that the ability uses twice the character's class level, minus three, for something, because that is significantly more rare.
Unfortunately, while the progression for each column of the table is inductively clear, only the BAB, Fort, Ref, and Will columns have explicit progressions in the textual portion of the rules. This means that while the stats for EDL 37 character using a 1st level animal companion very probably should be: 37 HD, 27 BAB, 20 Fort, 20 Ref, 12 Will, 37 Skill Ranks, 19 feats, +24 Natural Armor Bonus, +12 Str/Dex bonus, and 13 Bonus tricks, the actual stats are subject to your GM deciding the progression should continue that way, except for BAB and saves which are determined by whatever your GM picks for HD.
I doubt the intent was for double stacking class levels.
Response to objection 2:
Well, we can't be sure about intent without a designer quote, and we don't have any on that topic. We can see that there are a few other native (i.e. double stacking from a signle ability) cases where double stacking class levels occurs more directly, for example a Spherewalker with no other spellcasting class at the time the Speherewalker levels were taken effectively gains two levels of spellcasting ability for each level in the class. We can also see, as mentioned above, several design considerations that seem to have been taken to make sure the unusual formula behaved well-- the ability comes online at level 3 which is exactly when the previous 'equal to druid level' ability gained at 1st level is equivalent to this newly gained ability. Were the ability to come online past 4th level there would be a shap spike in power when the ability was gained, and were it to come online before 3rd level it would actually be less good than just usuing druid levels (and in fact might result in a negative number), besides making the question 'do ranger levels less than four have an effective druid level conversion rate' matter. So I think there's a reasonable argument that this is, actually, intended.
Ultimately, though, it doesn't really matter if it is intended or not, though, it's how the text works out. You count your hunter levels as druid levels and ranger levels for some things and your ranger levels - 3 count as druid levels, too. So, for a small subset of things you might want to qualify for you count as 2*druid level-3.