The 978-/979- prefix of an ISBN comes from the fact that ISBN13s are a subset of European Article Numbers, which have a three-digit prefix. This fixed prefix is affectionately called Bookland. We can thus ignore it almost completely.
Large blocks of 10k to 100 million ISBNs are assigned by the International ISBN Agency to international organizations. Then, each country has their own method of subdividing their blocks into publishers.
The important part is that ISBN blocks are always assigned by prefix. For example, 978-4 is assigned to Japan, and Japan assigned 978-4-312 to one publisher. That publisher then assigns the articles 000000-99999, leading to an ISBN of 978-4-312-99999-X, where X is a checksum. The longer the country prefix is, the fewer books can be assigned within it. For example, while Japan has a space of 100 million ISBNs, Singapore has a prefix of 978-9971-, allowing only 100 thousand ISBNs.
