400 Mbps plan but getting just under 300 Mbps

We recently recieved an email from Xfinity saying 

We're excited to share that we've increased your download speeds from 300 Mbps to 400 Mbps.  (Xfinity app also shows this as out current plan)

However  Wired Speed Tests to https://speedtest.xfinity.com/ and https://www.speedtest.net/ show our speeds remain around 300 Mbps (same as before)

Modem signals seem OK 

Suggestions?