In the UI, go to a project and open the dropdown menu for different branches and merge requests. The sorting of those seems a little bit confusing (see attached screenshot). So after switching from 99 to 100, the list starts with 100 instead of the lowest value.
What you’re seeing is an alphabetical sort of PR key.
This has been reported internally in the past, without getting enough “traction” to actually get fixed (partly because we couldn’t agree on what the sort should be.)
As for the merge request view which is numbered, I would expect them to be sorted by the number. For normal analysis on different branches the alphabetical sorting is fine.