When in a shop and viewing an item, it shows the currently equipped item with a comparison of the stats (if the option is enabled). If you then hover over the equipped items in the inventory, the same stat comparison is shown (regardless of which item you hover over). It seems like the stat comparison cache (or whatever) should explicitly be cleared when hovering over equipped items or something.
(It is also somewhat irritating to have the tooltip of the equipped item on the left, right on the mouse cursor, whereas the tooltip of the item that the mouse hovers over is shown to the right - the opposite order would be more intuitive imho).