you can reproduce this on your machine! Just copy my "Check_Ver.exe" executable to a test folder on Windows 10. Right-click it and select "Properties". Go to the "Compatibility" tab and select "Run this program in compatibility mode for" WINDOWS 7. Click the Apply button and then OK. Run the App and it will display (as expected) 1024 on Windows 10 because it runs in Windows 7 compatibility mode ;-)

I think your customer did the same. He can right-click your setup.exe and disable compatibility mode.

Does this help?

BTW, applications have to use the GetVersionEx API. The Registry values should never be used!