The site founder is on record as saying that an individual's personal karma total is related to, but is not exactly, based on the net of upvotes on submissions

The site founder is on record as saying that an individual's personal karma total is related to, but is not exactly, based on the net of upvotes on submissions and of upvotes and downvotes on comments. In other words, it is possible that a personal karma total could have movement up or down even if there hasn't been any recent up or down vote on anything you've posted here, or the other way around. I think the system is generally working as intended, but there is a little intentional security by obscurity to prevent gaming karma, I think.