Its due to how those ISBN ranges were handed out - I think they probably gave a block like 978-53 (for example) to those countries, meaning the right to distributed ISBNs 978-530-000-000 to 978-539-999-999 and then later they ran out or had all subblocks distributed to publishers, and then they got a new block further away (so not 978-54 in my example) and therefore those blocks are not numerically close to each other and thus also they are separate "islands" in the hilbert space.