This is because the cylinders lack the older (and breakable) cylinder pins to hold the extractor. The ammunition cases (DUMMY rounds for bench checks) provide the pressure to hold the extractor properly positioned in the cylinder. (After all, the extractor is cut with dummy rounds in the charge holes so the ratchets are positioned to provide proper carry up with cases in the charge holes.)
Good information. It makes perfect sense, but I'd never considered it before.