I can't speak to all the studies on there because I didn't read them all, but I'm pretty sure they compared 2 mins with the electric brush to 2 min with manual.

People see that generally electric toothbrushes cause better outcomes and assume the overall ability to clean must be better

Because it's literally true. Better outcomes means better ability to clean. That's how it works.

Manual toothbrushes obviously can get the job done, but electric does it better