I'll try to explain. Just a moment ago I wrote a lengthy reply but that disappeared to cyber space before I sent it. Too bad, it was a good one.
As you probably well know the prop lever controls engine RPM by adjusting the propeller blade angle instead of telling the engine to run at certain speed. For example if the engine RPM is higher than set (set with the prop lever) the propeller governor (which actually is the one you are adjusting with the prop lever) increases the propeller blade angle to increase air resistance which slows the engine RPM back to the set value. Of course when the engine RPM is too low the governor decreases the blade angle to reduce the strain on the engine letting it rotate more freely and hence increase RPM back to the set value. These are the basics.
Now when the propeller hits the highest blade angle stop (also called coarse stop), which for example happens in a high speed dive with power on, the governor cannot keep the RPM at the set value as it is unable to increase the propeller blade angle anymore (as the blade angle is at maximum already). Prop lever doesn't have any effect in such a situation which is why you need to decrease power by throttling back to keep the engine from over revving.
So in short the same stuff. The prop lever cannot control the RPM if it doesn't have the means to do that.
I think the recommendations to set lower RPM for dives is simply to give the pilot more time to notice and react to the governor loosing its capability to keep the engine below redline.
Hope this made sense and sorry for the lengthy reply :) .