Different things, the ethernet port, is 100 Mbps, but full duplex, the 300 Mbps of PHY rate is the maximum speed of the packet, but consider wifi overhead, retransmissions ... and moreover the wifi is half-duplex. For the
old 802.11n, you may expect, in good conditions, an actual throughput as 60% of the PHY rate. Below, a speedtest, between two AirMax N devices, wifi only, one way
Then, another speedtest, same conditions, but tx-rx at the same time
173 Mbps of total throughput, which is less than you can achieve, on a 100 Mbps full-duplex link (around 190 Mbps)