I upgraded my plan to 1.2Gbps internet speed but I am only getting 930 Mbps. Is this normal???

Recently, I upgraded my plan to Gigabit speed and rented a modem from Xfinity (XB7 3rd generation) but I am not getting enough speed for my plan. You can get 900Mbps speed paying $10 cheaper than my plan so I don't know why I have to pay extra... Someone help me please!