There seems to be only one public test of the Macmini9,1. Sometimes GeekBench is not accurate on the clock speeds. Take the clock speed with a little skepticism until more results are posted. I can't see why the system with the best cooling would be 3.04 GHz instead of 3.2 like both the MacBook Pro and Air.
Yeah, that's odd. I figured it would be clocked higher as it can probably have the best cooling of all of the M1 devices.
It might be a lower-binned part that can't run as fast. Assuming the best chips will go to the Pro.