For a mathematical explanation of net metering, please look up 'grid outputs' in the index of the Help system. The punchline is this: without net metering, your power bill is equal to purchased_energy * power_price - sold_energy * sellback_rate. With net metering, your power bill is equal to (purchased_energy - sold_energy) * power_price. Does that make sense? Your "net purchases" over the billing period are equal to the purchased energy minus the sold energy. To be a bit more precise, the equation I gave for net metering applies only if the net purchases were positive. If they are negative, meaning you were a net generator over the billing period, then the power bill is equal to (purchased_energy - sold_energy) * sellback_rate.
That's zero if the sellback rate is zero, which is normal, or negative if the sellback rate is nonzero. The sellback rate applies only if you produce more than you use.