Since this is the second time I've mentioned Bowker, let me just say that I do not and have never worked for them or with them, although I did meet with their reps several times when I worked in a different part of digital publishing. It's just that they're inescapable when you're talking about ISBNs.
There are regional ISBN agencies. The US agency, Bowker, assigns ISBN prefixes by publisher, and publishers assign within their prefix as they please. They're supposed to use one ISBN per edition and format, but many publishers use ISBN as a kind of SKU so you can't 100% count on that.
If that sounds sloppy...I went to publishing conferences fairly regularly from the late 90's into the teens, and I never saw a program that didn't have at least one session or panel titled something like "Publishers must improve their metadata practices."