Constancy is supposed to be a state of no changes. Something to be constant is to be unaltered in any way, by anything. That’s why, in math (and other areas), there are things called constants, that generally are specific numbers that never change. Pi, which is 3,1416… is a great example.
But when we use the word (at least in Spanish, I don’t know if it’s used in English this way), constancy means to do something regularly, in a daily/weekly basis or something like that. The “unchanging” thing is that we keep doing it. Like sleeping, eating. Those are constant actions, we’re being constants about needing to do those things. About doing those things.