10, 15, or 30K?

Maybe this belongs in the bugs and glitches thread, but I was wondering if anyone else had noticed this.

Sometimes while waiting the 12 hrs to use my friends birds, I will play earlier levels to farm for potion ingredients, and sometimes I will play levels with fewer than the standard three birds, just to see if I can three star them. On some of those occasions, I only receive 10,000 points per bird (assuming they complete it without sustaining damage) and others the 30,000 point bird allowance is divided among the birds used, either 15,000 each for two or 30,000 for one. I cannot figure out a pattern as to why there is sometimes a difference though, as it can happen two different ways on the same level played back to back.