Each major version of Lightroom has its own serial number. It would nbecessarily be different from either of the other serial numbers that you see. Since Lightroom didn't prompt you for your serial number, it was still stored in your computer system files. As long as Lightroom is working normally, there is nothing for you to be concerned about.
It's always a good idea to register new versions of Lightroom with Adobe. If, in the future, you need your serial number you can always log into your account history and obtain the serial numbers from there.
