Because there isn’t a 7.73 version of Virtualmin. It is version 7.7, package release 3. This is a problem we’re working on. The -3 should only be for packaging changes, but Ilia didn’t want to make it a real new version (e.g. 7.8 and 7.9, I still don’t know why), so I had to roll patches into a -2 and then -3 update.
We’ll be switching to semantic versions (e.g. 7.7.3) at some point in the not too distant future, but all of our tools have to be updated to handle the more complicated version math.