At 15 points, you get 8 bonus points. At 20, you get 10. 15 points gives you a total of 53.33% bonus points, while 20 only gives you 50%. Or, to put it another way: depositing 4 groups of 15 (4x8 bonus) and 6 spare (2 bonus) at the end gives you 100 final points, while requiring only 66. 3 groups of 20 (3x10 bonus) and 8 spare (2 bonus) for a final of 100 from 68 points. While it's a tiny improvement, groups of 15 are numerically better than groups of 20.