It more than likely only transfers at 1gbit between the modem and your PC. The data comes in at 300mbps on the cable and goes into a buffer. The data is then taken from the buffer and sent to your pc at 1gbit. Since you pc must wait until the buffer is refilled to get more data on AVERAGE you will get 300mbps on your pc-modem connection even though what it is really doing is running at 1gbit and then sitting at 0mbps waiting,.