There is a possible explanation. Original version installed in your machine was Windows 8.1 Home, then upgraded to Windows 8.1 Pro with BitLocker support. You accidentally lock the disk. The clean installation of Windows 10 detects the original Home key in the BIOS and proceeds to install the Windows 10 Home version. If this is the case, you should be able to then upgrade to Windows 10 Pro for free, should you need the extra features.