That's normal in some environment, especially if your client is not in a direct line of sight with the router at a short distance.
In my home I _NEVER_ had WiFi stay at maximum link rate regardless of what brand, chipset, setting, 2.4GHz / 5GHz, 802.11g / 802.11n / 802.11ac standard and cost of routers and WiFi clients.
To achieve potentially better throughput, change your WiFi client to BCM94360