400mbps plan -- getting 100-250

Per the subject, have a 400 plan, getting 100-250 on my desktop. Some background:

I have my own modem/router (Arris Surfboard SBG10 DOCSIS 3.0) that I've confirmed is compatible with Xfinity. I used it in NY (Optimum) before I moved to Phila 4 weeks ago and had no issues. Since I've moved here, my speed has fluctuated between 100-250 mostly, with rare occasions getting up to 400-475. Have factory reset the modem, tried buying a different one, had Xfinity come and replace the hardware in the wall. Same outcomes. What the hell is going on?