Hrm, interesting. I tried to find the mismatch with some searches but I came up empty. So I can reproduce your issue, but it annoys me that I can't figure out what the PQ-splitter is missing. :) I'll leave my searches here to make it easier to trouble shoot this.
Geocaching.com search for Troms og Finnmark, caches placed before 2013-05-07 gives 1001 caches.
Project-GCs MapCompare for Troms og Finnmark, caches placed before 2013-05-07 including disabled gives 1003 caches.
So, looks like 2 caches have been archived that Project-GC doesn't know about yet. None of this explains why the PQ splitter would say there's 989 caches in that span tho. I assumed the issue would be that a CO have selected the wrong region on a series of caches (common issue), but I can't find any cache that supports that. I can't find that any of the caches have been archived and unarchived.
Checking the numbers for the whole area the PQ-splitter says 8468 caches, Project-GC's Caches Per Area says 8504, Geocaching.com says 8504. That's a bit to big of a difference to make sense. It's also not that newer caches have not been picked up yet because running a search for caches placed after 3 November 2020 gives 630 caches on Geocaching.com, same number as the last PQ in the PQ-splitter.
Now I'm curious about the issue so I hope it can be looked into. :)