Their computer systems aren't horribly current, so my guess is that they can only input one delay reason at a time. So, once the diversion because of weather happened, it removed the mechanical delay. The mechanical trumps weather every single time.
Also mentioned above, I would never take the word of a station agent (especially someone in an outstation) that said they would take care of it once the flight is airborne.
AA's LAX customer service is usually pretty decent, but then again I frequent the AC more than the main floor so maybe that would make a difference.