Read this strip at gocomics.com.
I think this strip marks the end of the checkers winning streak series. Poor Charlie Brown should have quit while he was (less) behind.
This strip implies strongly that all of Lucy and C.B.'s games have been against each other. If the two tried playing against other opponents, maybe it wouldn't have gotten to 10,000 games? Of course Lucy prefers it this way.
It's like Paper playing against Rock.