For reference: [http://www.engadget.com/2013/07/25/mac-101-finding-your-macs-uuid|Mac 101: Finding your Mac's UUID].
Sadly the UUID is not the same value as your systems serial number it is generated using a few different methods depending on the systems age and firmware level. Here is a good write up on the spec: [https://en.wikipedia.org/wiki/Universally_unique_identifier|Wikipedia - UUID].
So it appears your systems logic board was swapped out at some point with a clean board. The tech forgot to burn the S/N or didn't have the needed tool (access) to burn it. Did you pop off the bottom cover at take a look on the other side? SOP was to make note of the new number if board was replaced.