it is a little plus extra, which do not really happen often. The chance that this do happen is under 0,1%.

when u look in details of the group mission which u choose u can see the etxra which could happen.

it could be a fight against Ginkaku = brings u an extra stone for ur waepons

it could be a fight against the white wolf = brings u extra points

or

it could be a fight against Fokurukomaru = brings zu double coins extra