First off, your account must be reflecting incorrect information. Extreme 105 is 105 mbps plan. Any rented DOCSIS 3.1 XB6 would still work with those plans but that’s a bit overkill, you don’t really need an XB6. A DOCSIS 3.0 modem/gateway at least. Even an XB3 would suit for that plan as well. And beware that there are some CMTS that has a bug with DOCSIS 3.1 devices that could slow your speeds.
FWIW, at one point I had heard that the XB6 gateway device was only being rented to customers that were actually subscribed to the Gigabit Speed Tier. Can an official Comcast employee chime in here and authoritatively confirm or deny please ?