The amount is the same because every time you prestige you get 10% more cash.
Prestige 1: 10% more cash, 5 + 10% is 5.5, this is rounded up to 6 because you cant have half of a number of gold
Prestige 2: 20% more cash, 5 + 20% is 6, which is your reward
So when you get to prestige three you will have 6.5, this will be rounded up to 7 so you will get 7 gold per kill.
TL;DR prestige cash is increased by percents instead of gold, so a low number like 5 won't change as quickly as something like 150 from a brute.