"slow" internet download speed

I am suppose to be on the 1200 Mbps plan and yet when I run a speed test (thru Comcast or another site), I only get about 280 Mbps. My PC is directly connected to the router (Asus RT-AC5300) and my modem is an Arris TM822. No one else was using my internet connection when I ran the test so I am mystified as to what may be the issue.  I appreciate any offered help beforehand.