It's worth selecting a few other target servers that look trustworthy, and to try a few other online services even if they are a bit noddy, such as fast.com which could be unreliable at the high end, but you won't have that problem. Run two different tests simultaneously to see if they sum to 100 Mbps.
You could also test the PC via WiFi hotspot to the mobile. The results of which will be interesting to interpret.
The most recent such report seen suggested it was a Windows issue that disappears testing with Linux.
A search online for a Windows 100 Mbps issue gets a fair number of hits, e.g.
Most forum users probably run their tests by browser or mobile app, and very few by the Windows app.
Presumably you've set multi-threaded test to maximize bandwidth usage and not single threaded to mimic a single file download?