I think they relate to specific values as well. The highest determines the descriptor, but its also impacted by exactly how high that number is.
I've been breeding Carnivine for the past 3 days trying to get one that has great EV's. One parent had 30 Attack, and the other parent had 31 Sp. Defense. The descriptors were "Likes to thrash about" and "Somewhat vain", respectively.
Every single time I got a Carnivine that recieved the 30 Attack IV, it was "Likes to thrash about". Every single time I got the 31 Sp. Def, it was "Somwhat vain". It tested this on probably 25+ pokemon over the course of my breeding, and it was always the case. Eventually I started immedietly releasing every pokemon that didn't have one of those two, and near the end I started releasing every one but "Somewhat vain" because since Sp. Def is higher, a baby with both the Att and Sp. Def IV would be vain.