Why can Pokémon appear at higher levels in Y?

I was looking through the location guide on this website, and it says that some Pokémon will appear in both versions, but at a higher level in Y? For the example, on route 21 in X, Dragonair only appears at level 50. But in Y, it appears at levels 50-55.

Pleaselog inorregisterto add a comment.

I tested this out and it seems that this actually isn’t the case. On top ofthis Bulbapedia pagestating otherwise, I tested it myself in Y. I used a level 57 Skarmory with Keen Eye at the front of my party, (assuming that Keen Eye’s outside of battle effect works while fishing, this should have made it so I would have encountered Pokemon 52-57 half of the time) and fished with the Super Rod. All of the Pokemon I encountered were level 50.

UPDATE:As sumwun suggested I do below, I put a Hustle Zweilous (in case it’s important, it was KOed and on level 59) at the front of my party. All Pokemon that were fished up were level 50.

I honestly don’t know but my prediction is that Game Freak is encouraging players to buy both versions. this applies to oras, sun, moon, ultra-sun and ultra-moon.