Ah, the fact that I was beginning with Ozzie I threw me off, and I didn't get a proper difference (should be 33 - 21 for the Guardian side, 8 - 1 for the Mystic side). I have an embarrassing weakness when it comes to subtraction.
Yeah, that would make the Mystics even longer-reigned than I calculated at first.
Now things make more sense. So originally, Square Enix having Ozzie XIII in 1000AD Medina implied that the rate of monarch turnover(a quantity ambiguously related to monarch lifespan) in Medina was on par with that of Guardia. Going to the trouble of changing Ozzie XIII to Ozzie VIII in the final might imply that whoever made that decision wanted to communicate one of several things:
1. Greater standards of living in Medina compared to Guardia over this period, yielding longer lifespans (whether due to magic, trade, etc)
2. Better protection of Medina monarchs compared to Guardian monarchs over this period (again, "magic" is an easy explanation)
3. Different political systems -- some kind of democratic element in Guardia's during the period would promote faster monarch turnover, for example. However, monarch election in Guardia strikes me as unlikely, seeing as Marle is descended from Leene, implying bloodline succession.
That's just considering the Ozzie XIII > Ozzie VIII switch. Getting back to the more general topic of standards of living, we could bring into evidence the kind of domestic furnishings available to Medina and Guardia in 600AD and 1000AD. However, I think in 600AD all we have access to are Magus' castle and Ozzie's fort, huh?