Multiple data usage monitor programs are reporting different numbers

I'm stuck with a data cap, unfortunately, and the ISP offers no way of tracking data usage. In an attempt to avoid going over the cap I installed both GlassWire and NetSpeedMonitor to help keep track of my usage.

They report wildly different numbers for total data usage over a given period. Both started tracking for October at the same moment but the number GlassWire reports is only 81% of the number NetSpeedMonitor reports. The difference will add up to multiple GB very quickly, so I'd like some ideas as to what could be causing it.

Thanks, and have a nice day.