The office building I work in has three elevators; my observation has been that when I call an elevator at high-traffic times such as lunch-time or 5PM I might

The office building I work in has three elevators; my observation has been that when I call an elevator at high-traffic times such as lunch-time or 5PM I might get any elevator while at lower-traffic times I tend to be allocated the left or middle elevator. My mental model is that an elevator with scheduled destinations (buttons pressed inside) is 'allocated' and an elevator travelling to service a call is 'allocated', while other elevators are 'free'. If you press 'down' and there are no elevators above you going down, or if you press 'up' and there are no elevators below you going up, the left-most free elevator will be dispatched. It makes sense to wear out elevators unevenly: If two or more elevators were evenly used, it's more likely they'd break down at the same time, leaving your building with no elevators at all.