Bonus Cards

can anyone tell me the difference between Flat Rate and Escalating Bonus Cards? 

Conquer Club, a free online multiplayer variation of a popular world domination board game.
https://www.conquerclub.com/forum/
Bonus Cards
Escalating sets are worth 4,6,8,10,12,15,20... (i.e. the first set of cards turned in by any player is worth 4 armies, the second set is worth 6, etc.)
Flat Rate sets are worth 4 for red, 6 for green, 8 for blue and 10 for mixed.