Bitcoin Core is the name of the software that was originally released by Satoshi Nakamoto and has been continuously been developed ever since. It is the most widely used software to interact with the Bitcoin blockchain, which makes sense, since at the beginning it was the only client and essentially the reference implementation.
The confusion stems from the fact that it was renamed at one point to make it clear that "Bitcoin Core" refers to this specific software whereas "Bitcoin" refers to the protocol and blockchain more generally. This distinction wasn't originally needed, as there were no other clients.
It does seem to follow Open Source best practice: but I'm not even sure what "decentralized" development would look like?