Why Italian frogman raids were different from ordinary sabotage

Why were the successful Italian frogman attacks different from ordinary sabotage?

They were planned Regia Marina special operations, while covert sabotage and German frogman attempts are described as unsuccessful.

Covert sabotage against the Royal Navy appears to have achieved little because crews were alert and suspicious. The Regia Marina's successful frogman attacks belonged to a different category: trained naval personnel entered enemy harbours on planned operational missions. Their success at Alexandria showed what a prepared specialist unit could do. German frogman attempts are also mentioned, but they achieved little.