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#31
Illegally, most likely. My best guess is he or she bought it used and it either had a corporate Enterprise install, that should have been wiped before resale, and will eventually fail to activate. Or its an illegal install that was activated illegally with a fake KMS server. Enterprise will look like Pro and if you don't go and check the system page you could think its Pro. Pro even has Windows To Go now.
EDIT: Looking at that key its likely the latter.
Ok, so whoever you bought it from, put Windows 10 on there illegally, either because it was previously used at a company with Windows 10 Enterprise and they neglected to wipe it and reinstall the original OS, or by installing and illegally activating it against a bogus KMS server.
Since you have a Windows 8.1 Pro key already in your BIOS, you should be able to just install Windows 10 Pro, and it will recognize that key and install and activate Windows 10 (even though the official free upgrade period is over, it's reported that the free upgrade still works) If it asks for a key, just click "I don't have one"). If it asks you which version to install (I don't think it does if you have the key in BIOS), then make sure to choose pro.
You may be able to use the registry tricks others have mentioned to do an "in place upgrade" and trick the install Pro over Enterprise, but if it were me i'd just do a clean install.
i change registry to win 10 pro as #2 and reinstall win 10 pro from windows site. after install it show win 10 enterprise. i go setting/about/change product key and use OEM key. problem solved