Hi Edgar,
The RSSI is the Relative Signal Strength Indicator of the signal that is received. It does not tell anything about the quality of the signal. There might be RF noise on top of the signal that makes it less desirable for the receiver. They scan the individual signal and compare it with the signal criteria needed for a good signal. The different criteria are then weighted by an algorithm that has been figured out by some smart frequency guru. The signal is selected after these criteria are applied. So it is not necessarily the stronger RSSI signal that gets the choice. It is rather the cleaner signal that gets the choice. I could go on, but that is getting into too much detail.

Thanks,

Gerhard