[c-fe]

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.

[matsemann]

I see, thanks for explaining. Cool that your visualization then shows these idiosyncrasies!

[c-fe]

Thanks! That is indeed all thanks to using the hilbert curve fractal which has the property that it maps numbers which are close together onto 2d (or higher dimensional) coordinates which are close together, its a very cool property! Its used in lots of contexts for that reason