App updates from different sources

I'd like to know why F-Droid shows updates before Google Play Store. Typically these are apps like Hacker's Keyboard and NetGuard Firewall, etc. Wouldn't the developers of such apps submit changes at the same time to both? F-Droid also tells me that the signing key is different as well. Is it a common practice to create new signing keys upon updating an app? (Yes, I'm concerned about 3rd-parties using dirty tricks to gain access to my devices.)