Contributor value decreases. Why?

Did you try searching the forums? Lots of people ask about it decreasing.

Unfortunately, if the price of a Steam game is permanently lowered (not a sale, a permanent price cut), the Steam Gifts value is changed to reflect the new price, even if you gave away the game before the price cut.

I think in your case Ghostbusters: The Videogame has been lowered from 20 to 10 - do you remember the Steam store value at the time you gave it away?