Different download speeds using same modem and CAT 6 cable

Hello,

I'm getting vastly different download speeds on three computers.

Computer 1 - Dell Optiplex desktop - 440 mbps median, min 19 mbps, max 791 mbps over the last 18 months

Computer 2 - Dell Inspiron - 10-20 mbps

Computer 3 - Dell Inspiron - 30-40 mbps

All computers are hardwired to my brand new rented ($25/month) Xfinity cablemodem and are hardwired with CAT6 cable.

I'm paying for Gigabit service ($116/month).

I am not, repeat, not getting anywhere near gigabit service.