AirTime 300Mbps under optimal conditions equates to roughly 100Mbps cable...In other words if you can get a link of say 180Mbps between AP and STA, real world airtime throughput is about a third (i.e. 60-80Mbps).

It's all about AirTIme. To get 100 Mbps over air, the AP must send the data much faster to keep up with this speed, b/c it can't allocate the channel all the time and b/c encapsulation in radio frames adds some overhead, too. Real data speed is roughly 65% of wireless speed in 802.11n mode, so it perfectly makes sense to send data much faster over the air than it comes in on the Ethernet port.