XB7 slow speeds. 1.3Gb to 1.5Gb to the modem but 950Mbps on the ethernet cable?

I just swapped my XB7 with a new XB7 with the tech that came out. I pay for 1.2Gbs and testing from xfinity shows I'm getting 1.3Gbs-1.5Gbs on site and from phone support. Testing from the tech and my hard wired pc shows I'm getting 950Mbps output from the modem. Supervisors and Tier 2 support HAVE NO CLUE why the modem throttles our speed. Is it a setting at comcast??? Some people on here are getting 1.4Gbps with the same modem. Why is xfinity throttling some of our modems???  Its 100% not my wiring or devices since we tested before the devices.  Thanks.