FWIW you don’t actually see the seniority numbers changing on a monthly basis (due to people leaving), they only get recalculated/reallocated once a year.
The official number (that’s used for fleet changes etc) only gets updated once per year, but there’s a third party app (rhymes with “ShyKid”) that uses the monthly bid pack and some voodoo magic to guesstimate your current actual seniority. I’m led to believe if you’re in the bottom 2/3rds or so (as most of us are) it’s pretty accurate.