The original project was bought by a company known for injecting ads into their apps, so this fork was made by one of the original maintainers in order to keep it ad-free. This is just off the top of my head, so go look into the project if this interests you.
The OG version is no longer FOSS and was sold to some shady company. So far there doesn't seem to be any changes to the old one yet but I'm sure they'll creep in sooner or later.
Seems like there's been some small improvements to the new version too when it comes to the look and feel.
Edit: just refreshed the comments and all of a sudden 3 other people already explained it, all 30 mins ago? Huh.
In regards to your edit, federation means sometimes updates are less synchronous than immediate. Some server somewhere got hung up on the way to serve you that data; maybe your own instance, the OP's, or some third instance or multiple out there in the interwebs. I don't have the technical knowledge to explain beyond that though.
The way I understood it, the original changed ownership to some shady types. This triggered some concern that has led to the fossify forked versions.
As far as I know there haven't been any releases of the original since the ownership change, so this is a bit preemptive. I prefer that to finding out too late, though.
Currently nothing, fork even removed panorama library (idk what it was) and advanced photo editor (which was good). I switched just to stay in touch with development, as Original won't be updated and I do not use editor anyway.
This is open-source at its best: the original developer sold out, and the community re-routed around the newly-created enshittification problem
Not to mention, when ZipoApps realize nobody downloads their tainted apps and their investment is essentially worthless, they're gonna be so mad! I'm not normally a petty man, but whenever it's one of those hateful ad companies getting the shaft, I feel pure, legitimate and totally assumed schadenfreude.
I'm eagerly waiting for Fossify Calendar - the non-sellout version of Simple Calendar, and the only Simple Mobile Tools app I use.