Reforging any item currently has the stat increase duplicated for every gem with the same stat in that item. These extra stats are not shown in the tooltip.

Example: Reforge an item to add 26 haste rating.
If the item has no gems with haste, you correctly gain 26 haste.
If the item has one gem with haste (any type), 52 haste rating is added.
If the item has 2 haste gems, 78 haste rating is added, and so on.

I also tested this with reforging to expertise+expertise gems, and reforging to hit+hit gems, with the same results. I don't know if enchants also cause this behaviour, but it wouldn't surprise me ;)