I think it makes perfect sense. Think about it: most characters will want to level at least one of their alt classes up to get their cross class skills. Except for Paladins and Dragoons (who are done with useful cross-class skills by level 15), everyone else needs to level up to the twenty or thirties. Warriors will want Provoke from Gladiators (lvl 22). Monks and Bards will want Invigorate from lancers (lvl22). White mages, Summoners and Scholars will want Swiftcast (lvl 26). Summoners and Black Mages will want Quelling Strikes (level 34). And let's not forget that a player can have all classes in one character.

As you see, there will always an higher probability of level 15-26 dungeons to appear in the roulette.
Ifrit Normal Mode can be explained because of new players and old players making alternate characters. Those people will want to defeat Ifrit NM in order to unlock the Grand Companies and be able to get the mount for their alt, even if they don't plan to stick to it or play it much.