Unexpected Activation !

Hello All, I have just successfully installed a clean W10 Home via the Media Creation Tool.
The PC had W7 installed and prior to that Ubuntu, and prior to that W8.1 !
Apart from a minor hitch [old BIOS] all I had to do was to switch from MBR to GPT as the install was UEFI.
I was all ready to input my brand new W10 OEM Windows Home key [recently purchased with DVD] but was never asked for it and when I checked I found W10 showing 'Activated' !
It's not my birthday so can anyone tell me what has happened [or what might happen] ?
Thanks,
MWombat

Thanks Dalchina,
Isnt that interesting ?
Have just run ShowkeyPlus and now I have all sorts of key info. Interesting in that the output refers [in part] to the original W8.1 OEM [probably the only OS apart from the new W10 that was legitimate !]. This is a surprise to me in that in prepping for the W10 install I had to update the BIOS [GPT needed] so I thought that any reference back to 8.1 would have been zapped.
Anyway it looks like good news though in that I didnt even get as far as 'scratching' to uncover the new Key - I will save it as I still have another very good PC still running W7.
Thanks again,
MWombat

MWombat said:

...just run ShowkeyPlus and now I have all sorts of key info. Interesting in that the output refers [in part] to the original W8.1 OEM [probably the only OS apart from the new W10 that was legitimate !]. This is a surprise to me in that in prepping for the W10 install I had to update the BIOS [GPT needed] so I thought that any reference back to 8.1 would have been zapped.

No, the bios code you just updated and the embedded OEM W8.1 key are stored in quite separate sections of the firmware. And that W8.1 Home key entitles you to activate W10 Home.

Yes, that makes sense.
Otherwise whenever anyone 'flashed' the BIOS the PC would lose any reference to it's OS key.
Seems like I have had a win !
Thanks everyone,
MWombat