When adding a progression to a custom codex detail, the sorting of the details is done from which details you've added first.

So when I added Backstory and Communication, they were the first two (aside from list selectors (like age, and gender)) and then I later numbered them (as right now you can't reorder the custom details).

Although numbering works as reordering on the codex entry itself, it seems to not take effect in the progression addition target section.