I think cgminer is retrying all shares that it thinks it previously failed to submit. It does not report them as invalid yet, because it has not yet given up on them. When i look through the logs, I see you submitting boatloads of 'unknown-work' shares, each for a different block. These are all very, very old shares that cgminer is constantly re-submitting. The original submission attempt might have even been successful. I suspect cgminer wants to see a certain kind of error message before it gives up, and that ABCPool is sending a different one. Each resubmission counts as one invalid share, although in most cases it's the same shares over and over again.

In the next release of our backend the error messages have been changed so they look more like those of the original pushpool. We are still testing this version, and expect to have it ready in a few days.