Expecting 100Mbps upload but getting 20Mbps

I am expecting 100Mbps upload speed as advertised for my address on https://www.xfinity.com/broadband-labels, but I'm seeing uploads of 20 Mbps.

My download speed is fine.

I just signed up for service with my own 32x8 DOCSIS 3.0 modem on the list at https://www.xfinity.com/support/devices/#auth.

The automated troubleshooting shows no problem and the internet is otherwise fine.

What might be the issue?