am looking at my walls now and i have 62k LS ''defeated'' as a defender that should give me 620k DBP only for that part. we never found out how does the BP sharing work so i guess this stuff, and many other weird ones, are possible
Few things i have noted about the bp sharing:
owner of city gets a larger cut, as does owner of siege.
For the supporters it seems to work in this format: it counts the total pop of troops sent, be it land def or br. The person with the most population points of units seem to receive a larger cut than rest of supporters.
Also you have to factor in, the person's city your sieging gets a percentage of the bp you earn defending the cs from the break attempts.
Have not worked out the exact formula yet, too many variables to test.