Nah, I would guess that they have good growths because they liked them or something.
By that logic, shouldn't Sakura's subordinates Hana and Tsubaki have growths as good as theirs? Hana's growths look decent, but not as good as theirs, and Tsubaki' seem pretty bad.
Speaking of Tsubaki, can anyone tell me if he is as bad as he looks? He's definitely one of my favorite characters from Fates, but I was really disappointed in his growths and stat mods. That 35% speed growth kills me...