Dents don't spread but cracks do.
The limits in the maintenance manual are based on engine tests and stress measurements. There are vibratory stresses in the running regime that could progress a defect into a crack and fail the blade. These stresses vary by engine model and rating (as the Kegworth incident demonstrated) and also vary by span location on the blade. Thus the dent/nick/gouge limitations in the manual also vary by span location.
OH, and don't forget about the possibility of a bird ingestion on just such a blade. Prior damage doesn't mix well when the blade bites into a bird.