So following issue happended: I bought an Artificer's Hammer on the community market with 15 charges.
After I removed ONE Gem in game, the counter decreased to 4.
I found a post about the same bug on the community forum, but none here.

I am not quite sure if the original number of charges was just wrongly displayed, so I dont wanna blame some one in particular, but how everyone can imagen, a hammer with 15 charges is more expensive than one with only 5!