Why is my new MB8611 slower than my MB8600?

Per Xfinity suggestion, I upgraded my modem from an MB8600 to a MB8611. Before I replaced modems, I tried my connection using speedtest. Download Mbps was 375.18 and upload speed was 5.86. After installing the new modem, it was sub 300for download and 5.41 upload. I tripled checked to see if the new modem was indeed an MB8611, and it is. Any idea what's happening