a lot of Gen 5's evo levels make sense within the context of BW/2, where XP is plentiful and you end up finishing the game at a much higher level than in most other games.
The problem is when Unova pokemon are brought over into later games that don't have the same level curve. They should've just edited evo levels to better fit the games where the pokemon are used over again, but they don't. Mainly because GF has severe autism about evolutions and not changing things (don't even get me started on all the evo methods and bullshit in Gen 4 onwards rather than just using evo stones)