2.4 ghz WiFi running about 100 Mbps well below modem-router limit of 600 for 2.4 ghz

It was suggested I make a new post for this issue which I originally had as a reply to another post:

My son and I both have 200 Mbps service and have MG7700 modem-routers.

The 5.0 radio runs okay at around 200 Mbps.

But the 2.4 runs under 100 which is far under the 600 max for this modem.

Wi-Fi Speed and Performance
• Simultaneous Dual Band radios
• Up to 600 Mbps** at 2.4 GHz
• Up to 1300 Mbps** at 5 GHz

** Speeds shown are maximum wireless speeds derived from IEEE Standard 802.11 specifications. Actual speeds will vary.

I also have Motorola MT7711 which is a similar gateway that also only reaches 100 Mbps on the 2.4 radio. 
That other post I mentioned was by someone with 400 Mbps service and his 2.4 likewise is limited to 100 Mbps.