Here's my guess:
Notice on the character page, every time you changes a piece of armor or weapon, the armor ID will change. So if you wear 1 set for a long time the message might appears, the chance of upkeep will be reduce.
So when that message appears the chance of paying upkeep will be reduce by a little bit for that particular set.
I would recommend mix and match armor and weapons to total of no more than 25k, or you'll start losing $$. Remember to account for loomed items prices are much higher but upkeep is the same. Weapons prices are too high imo.