Mnnh - SWA, particulary with regards to aircraft records, is getting "the fisheye" from the FAA at the moment. As well as a lawsuit, both stemming from the fatal engine explosion near Philly. A time to make sure all the i's are dotted and t's crossed.
And the backstory is, the FAA itself is getting some flak and oversight (IG investigation) for perhaps being too "chummy" with operators.
"An abundance of caution," as the memo states.
As to how it happens - weight may "accrete" on an air frame, as things like passenger wi-fi, or even flight systems, get added or upgraded (or even removed). And maybe not all departments "get the memo" of the change to aircraft N000SW's weight, so the records don't match. Big corporate bureaucracy (or perhaps too small and overworked a corporate bureaucracy).