I think I can shed some light on this.
JNB alluded to it, and I'll be a bit more specific. The maximum takeoff weight for a given transport category airplane can be limited based on a number of performance factors. I won't get into the details unless someone asks, but
usually the most limiting factor has to do with how well the airplane climbs if an engine were to fail.
As you can imagine, the altitude and temperature at an airport can greatly affect an aircraft's performance. So a 50 seat RJ might be able to take off today from Phoenix fully loaded with passengers while still meeting the single engine climb gradient required by the departure procedure. But that same airplane in the middle of August, when it's 45ºC instead of 23ºC (like it was today), might only be able to meet that required gradient with 45 passengers onboard. Most of the time the airlines plan for this, but sometimes the airplane is a little heavier than they anticipated, or it was a little hotter that day, and they need to bump a passenger or two to make the numbers work.
So
dukebound85 isn't completely off his rocker.

You'll see this happen most often during Summer in the Southwest, where the airports tend to be at higher elevations and at high temperatures. PHX, LAS, ABQ, ELP, SLC, and so on...