Considering it was slower than the 604e and cost more than the 604e. It makes sense Apple went with the 750. The G3 was actually a pretty cheap computer. Especially the iMac.
I don't know much about the 620. It sounds like it was meant for a specialized market that needed a lot of RAM. Way more RAM than anyone would have needed in a single computer. Which I doubt would've been utilized very well in Mac OS back then anyways. What with it's horrible multi-tasking and multi-CPU support. More like something for Unix and Linux systems. Possibly IBM's mainframes.