The "not guaranteed" part is just a legal buffering maneuver to mitigate litigation due to how consumer electronics are and the medium by which the data is sent. My cable modem could probably do 1 Gbps easily if the modem's profile allowed it. Not only that, but I purposely set the modem on my work bench next to the wall where just behind the wall to the outside is the network interface service entrance. That will insure the modem is as close to the service entrance as possible to minimize any data decay through impedance on the coaxial line.
The missing 1.8 MBps
could be due to some unknown overhead in my router or computer. I could rule it out by plugging directly into the modem and see what happens. But because Comcast has so-called "sticky" IP address assignments, if I plug directly into the modem with my computer for testing, the MAC address will change and thus I'll get a new external WAN IP address. Then when I plug back into the router I doubt I'll have the same external WAN IP address I had before. This can be a real pain when your IP address is tied to a firewall and lots of other things for access control that can't (at this time) take advantage of a DDNS issued name.
Next thing you know pharmaceuticals will use litigation mitigation by stating a whole class of "possible" side effects. Sure, I always wanted to take
Lithotripsy and have the overwhelming desire to sniff my carpet and oink like a pig... Oh wait.