Your 9th position switch on the 100's reel may be stuck closed or shorted for player 2. If that is the case, every time the 100's score reel fires, the 1000's reel will also. It explains the 100 -> 1100 issue, but would also cause a 500 ->5500 (not 4500) issue...
Search for "9th position switch" in the forum, a common problem...
Do you have a schematic?