AT Gun CC attack gets +10 modifier!?

[gordrob]

In the Monster tutorial battle, my Panzer IVD drives onto the same space as a Russian 45mm anti-tank gun. On the Russians turn, the gun attacks my tank in close combat. The log shows the Russian attack having a modifier of 10, but the details only show -2 for close combat and -2 for terrain modifier. How did -4 turn into +10?