New Modem vs Old Modem

Having internet stability issue, so decided to replace 5 year old Arris SB8200 with new Hitron CODA56.

Setting up the CODA was  a bit more painful than expected but now it is running OK.

Ethernet download speed with SB8200 topped out at 950MB vs Xfinity spec of ( up to) 1.1GB.

After the install, both modems are showing in my account profile with no apparent way to delete the old SB8200.

Only the new modem is physically connected, but if I replace the old SB8200 that also delivers 950MB.

So, I have 2 questions:

1. If both modems can be installed separately and deliver the same speed, does that mean the new modem is not correctly provisioned for best speed?.

2. Does it matter if the account shows both modems (only 1 connected) - is it worth removing it or just keep as temporary back-up in case the CODA fails.

Thanks