Pretty straightforward

I've enchanted items that increase pickpocket success by 40%. When I put them on, my chance to steal goes DOWN, more with each item used.

I don't know if this is true with pickpocket items naturally found in the game, but it is true with enchanted ones.

Can just give 2000 gold to an NPC for testing, and then watch your success rate to steal it back plummet when you equip +success items, if you need to reproduce this