Internet upload speed not as high as advertised

I purchased a 300 Mbps down and 100 Mbps up plan from Xfinity. The Xfinity speed test is showing 351.8 Mbps down and 21.5 Mbps up.

I have an SB6190 modem, which is rated for 1000 Mbps down and 240 Mbps up. I ran the speed test on a laptop hard-wired to the modem with an ethernet cable. There is no router in between right now. There are no other devices connected over coax in my residence.

Why is the upload speed only 21.5 Mbps when it should be 100 Mbps? Thanks