Here's the craziness at the end of the 3rd round.
https://streamable.com/em05
Herb Dean did everything right, he didn't stop the fight, and there is no reasonable expectation that he should have stopped it because Bisping was clearly saved by the bell, with the round ending as he went down. Bisping was also not unconscious. If Silva had just a few more seconds he may have been able to get the finish, but bad luck, he didn't have those seconds.