I compared the web page tester against the Windows 10 app rating both out of 10.
Web page tester: 9 (Acceptable: No way to report issues)
Windows 10 app: 1 (Terrible: Very Inconsistent test results, no Help About and no way to report issues)
Clicking GO several times in the Windows 10 App provides a high variation in results (download: 10-138mbs, upload: 14-18mbs). The web page tester provides expected results (download: 100-150mbs, upload 11-13mbs).
I suspect the implementation of the Windows 10 app was assigned to someone who knows about implementing apps on Windows 10 but less about handling network events and measuring throughput.
I rate the Windows 10 app as extremely poor -- app needs testing before distributing. To be a network distributed application it must have Help About with software version and provider and any legal notices, especially if there are privacy considerations.
I have uninstalled the Windows 10 app for fear of privacy violations.