One thing to keep in mind is that PSA started out grading sports cards which tend to be slightly larger and thicker than ptcg. I tried to breakdown the generations of psa cases in a thread last year and I found that pokemon cards were graded in the same sport card cases since the late 90s:
Sport card cases saw a number of revisions to their cases through 2008. It was also in 2008 that TCG cards received their own version of cases which have a backwards “21” in the lower left corner.
So what I suspect is that in 2008 there was a small window where TCG were no longer slabbed in old sport card cases and before they finalized the TCG specific cases. This is me speculating of course. As for that Arcanine, that card could have been re-holdered anytime after 2017.