Xb10

 Just hooked up my new xb10 modem/router, i have the 2000Gb DL/UL plan. Came from an xb8, when i did a gateway speedtest to the modem from the xfinity app i was getting my full DL speed of 2000Gb and only about 250Gb upload speed from the xb8, doing the test with the xb10, i am now receiving about 1200Gb download and 2000Gb. Why is my DL speed now slower? Does the modem need to have a certain fw or something?