You get one or the other. The computer "knows" you have the nav system and disables the duplicate screens. In theory it's based on the same data, but I suspect the module that snarfs the data from the CAN bus and sends it to the NAV Radio is a little slow and misses some bits from time to time. I've seen the instantaneous bar drop from MAX to zero then reappear a second later.

Also I don't know what happens calculation wise on the average if you leave the radio turned off.