Which Serial Number?

I am curious about the serial numbers provided by Stardock. I rec'd a serial number for SoaSE via email upon my online purchase of the game. I also opted to receive the game disc in the mail. When I rec'd the physical DVD it too included a serial number inside the case which is different from the original email serial #. I had already installed Sins on my desktop. The desktop contained the original downloaded Sins and the email serial #. When I installed Sins on a new laptop which I got last week I wasn't really thinking and simply entered the serial # that came with the DVD. This serial number worked fine. So is this okay having two serial numbers for the same game? Should I have only been using the original serial # that was provided?