After "Factory Resetting" my tablet I realized that the serial Number was changed (it is now a weird number 0123456789ABCDEF).
Edit March 17 2019
This is the number that shows when I do adb devices. As far as I know this is the serial number and not the Android ID.
Why did this happen? How do I go about returning it to its original SN? How do I prevent it from happening again?
I have seen some options of changing the SN number but I'm reluctant to try them since the programs they use doesn't look like official (safe) programs.