How ALPA gamed it in 2006 was something like this:

They took the average pay rate of all of the SOs, FOs, and Captains and used that as the way to gauge the percentage. So since most of the SOs and FOs were junior, all of the money went to the captains. Bottom line I was a 5 years SO and I got about 8k, FOs I was senior to got about 16K, and Captains got about 32k. So even though my hourly rate was 80% of same seniority FOs I got a bonus of about 50% of theirs. My memory is probably off but I doubt by much.

Look for similar shenanigans. Or talk to your seniority demographic representative.