There are several bits you could be questioning, so I will answer each in turn.
Originally there was the Mac Pro and MacBook Pro, both designed to be used in a professional environment. For example, they had the power to do professional video editing well when most consumer machines struggled. But now there is the iPhone Pro, which is not particularly aimed at work use.
The 5K iMac screen has a lot of pixels to drive, so the M1 is probably not the right chip for it, hence using the M1 Pro.
The M1 Pro has so far only appeared in a machine named Pro. A Pro chip in a Pro machine makes sense. So an M1 Pro powered iMac might naturally be called the iMac Pro.
The Mac Studio is not called Pro, but may well include an M1 Pro, breaking this sequence. However, Studio is a new naming convention, so maybe we will see something new with it.