how aswan bonus coins work.

I'm doing hard mode solo attack and I'm getting:

brown 10 silver 4
brown 12 silver 5
brown 13 silver 5
brown 14 silver 6
brown 15 silver 6

:c?

edit:
brown 16 silver 7
brown 18 silver 8
brown 19 silver 8
brown 20 silver 9
brown 21 silver 9
brown 22 silver 10
brown 24 silver 11

maybe party gives more coins? how many coins you got on your last run?