The OS detection uses the Windows NT version number, see https://msdn.microsoft.com/en-us/library/windows/desktop/ms724832(v=vs.85).aspx
We did have an issue where Windows 10 was being detected as Vista, this is fixed in 8.1R11 and 8.2R6 (PRS-344992).
I found an old PR where Host Checker was detecting one Windows 7 machine as XP while other Windows 7 devices passed successfully, can you try the following which was the fix in that case:
Step 1: Go to Host Checker folder in c:\users\\Appdata\Roaming\Juniper Networks\Host Checker".
Step 2: Right click on dsHostChecker.exe and go to properties.
Step 3: In Properties dialog box, select "Compatibility" tab.
Step 4a: Under "Compatibility mode" section, if the check box was marked with tick, then un check it and click OK.
Step 4b: If the check box was not marked, then select it and then select "Windows 7" in the box down box below and click OK.
After the above steps. Restart the machine and run the Host Checker again.