Tech says He's seeing 1967Mbps Download speed at XB8, so why am I only seeing 600Mbps at my Router?

Here's the story.  Any advice/guidance would be greatly appreciated.

I upgraded from 1G to 2G and had to rent an XB8 to get the upload speeds  (Another story).           Set up the XB8 in Bridge mode so I can use my ASUS router.    Connect my router (2.5G port) to the 2.5G port of the XB8.  I am only seeing 500-600 Mbps from the router.      Chatted with an Xfinity tech (Thank you Marco) last night who said he tested my line and it was excellent and he was able to get 1967Mbps when he connects to the XB8.  hmmmm???????     Previously I had my router connected to an Arris S33 modem (with 1G service) and was consistently seeing >1G download speeds, so I'm certain my router can handle it.   I don't see any settings on the router that should throttle the connection.  My router log shows that it has 2500Mbps full-duplex connection to the XB8.    Any Ideas??