Hum Burgurne it maybe because you used that board for the 8 and then have come back to 7??
What Noel says is right too because I had an OEM where the motherboard died in a desktop I was using and with a short call I was allowed to install it on one of my laptops that had a new SSD (was Vista). I actually spoke to someone I don't know what went on with your situation though.
If you are still concerned why not ask Microsoft direct as to how many times it has been activated if it comes back 4 times then you will know someone has had their hand in the till