Not getting advertised WiFi Speed

According to my account I'm supposed to have "Download speeds: up to 1200 Mbps" however I have never gotten higher than 475 -400 Mps. I've had service for about 2 months now and the wifi speed is pretty stable regardless if I'm far away from the modem ( ARRIS GROUP, INC. TG4482A from Xfinity) or holding my phone 6 inches away from the modem. Is this to be expected? Is there something wrong?