Another wrinkle: different pots of money for different delivery modalities. Where I am, online courses are paid for VERY differently from face-to-face ones. General-fund money pretty much can't be used for online courses, so a much higher percentage of instructor salary for those comes from tuition.
Of course, the mix of online and face-to-face courses for any given instructor varies from semester to semester, so... have fun with that math, I suppose.
Yet another wrinkle: grant funding, for instructors who also pursue research grants. Grants don't last forever, but they may "buy out" one or more classes for a given instructor for a while (at which point less of their salary is from tuition).