I use Xfinity with a gigabit plan, no fan of them for various reasons, but my "signaling rate" exceeds 1gbps at the physical layer in the cable modem and since

I use Xfinity with a gigabit plan, no fan of them for various reasons, but my "signaling rate" exceeds 1gbps at the physical layer in the cable modem and since I am using the 2.5 Gbps ethernet port on the modem my speed test slightly exceed 1Gbps usually 1.1 Gbps. If you use gigabit ethernet out of the modem/ont then yes you will not see full gigabit speeds due to overhead. This is of course dependent on the type of connection you have and the equipment they use, so your broad statement is inaccurate.