Pay hours aren't the same as flight hours. If you are senior and get awarded an 80 hour line, working a day off automatically adds 4 hours on top of your guarantee. Any hours flown on a day off are at 1.5x and added on top of guarantee. Any flight hours flown above your guarantee on a day on are added on top of the guarantee.
Being awarded a high hour line and working a few off days a month is all it takes to bring home big numbers. Nearly everyone in the past year has broken guarantee each month. This year will be interesting, I have a feeling that OT will be harder to get.