Network capacity seems to be a more theoretical number, more like a sum of Client TX / RX + the unoccupied airtime counting as if an ideal max rate client is using it.
For example, if you only have one weak client connected that's using 50% of airtime transmitting at 25mbit, that would be the Client Tx/Rx value, but there'd be an additional 50% of "capacity" * (450mbit for N, 866 or 1300 mbit for AC) available on the channel.