What is a 'variance' after all?

Is it 'set in stone' once we gather data from a sample? Yes and NO, yes in one instance primarily: when we look only at 1 variable , in 1 group only.
If we have 2 groups OR more than 1 variable, say males & females, we can compare the 2 variances, and that means in fact comparing a MODEL with the 2 variances different to another model with them SET equal. The 2nd model here will yield NEW [population] variance estimates for males and females, not surprisingly equal (WE set them =!).