Windows 7 key from a Dell machine on a new motherboard

Hi everyone. I've been reading up on Microsofts licensing schemes and I thought I understood them, but now I have a situation that I don't quite understand and I could use some help. So I got my hands on a used Dell PC that had a Windows 7 Pro 64bit COA sticker key on it. I also received the installation DVDs with the setup. I thought that this type of setup falls in the OEM licensing scheme and is tied to the motherboard. I didn't think the key would work on a new motherboard until I installed a new motherboard, a different motherboard (make and model) to the system and after reinstalling Windows 7 with the DVD, key activated just fine. I didn't think it would and this puzzles me. Why exactly did the key work? Is it maybe not an OEM key?

Machine was bought in Germany, and I'm from Croatia, if that makes a difference. I read online that EU has different laws and product key reselling was made legal or something like that, is this the reason why key worked, even though it's on new hardware (motherboard)?

I know, it doesn't make sense, but since it activated I guess I'm in the clear. I just wonder how will upgrade to Windows 10 work, since I hear that it somehow ties itself to hardware.