The only thing that makes this strange is that if you reset a skill to 15 , and want to take it back to 100, would this even be possible if there are no quests left. I mean there are not many quests when you have completed almost all content in game.
Beating up legendary dragons to gain XP is not fun any more at high level.
Also if you reset lets say your heavy armor skill to 15 does that mean you no longer have any of the perks. So your 350 Armor drops back to 150 or what ever it was before the increases.