I don't think the xp amount is particularly retarded since the cohort isn't usually seen as a full party member, but rather as a supporting character, no matter how hard they fight in battles, etc.
Besides, some DMs (like myself) see the whole system of XPs and Challenge Ratings as a kind of abstraction. I tend to run levelling up more on a system based on when PCs vanquish a certain group of foes and traps etc. My campaign is more of an accelerated system, so probably doesn't necessarily apply to others.