I think one important detail that's missing in the OP is the reason you would have not one but two reserved floppy drives. It was so that you could copy files from one floppy to another and duplicate floppies much more easily. Also, when installing games that came on multiple floppies, you could have one drive going while you switched the other one out. For this reason both A: and B: ended up reserved for floppies, as many people had more than one.