One of the things that happens in Arizona is the very low humidity and high temps are going to really dry out any lumber product - quite a ways beyond what kiln drying at the mill will do. My supposition is that the wood benchwork is actually contracting in the summer dryness, which would cause the track to appear to expand. In winter, when humidity is a little higher, the wood would expand to closer to its orginal size.
I could be wrong, too. There is about an 80 degree swing in temps in uninsulated garages in Arizona.
Regardless of what the cause is, there are going to be issues with a garage that reaches 120 degrees in the summer, and can fall to below 40 in the winter or night.
- it's not very comfortable for humans unless it's insulated. An air conditioning unit would help, but will actually worsen the wood contraction problem.
- painting lumber/wood is almost a necessity to prevent it from drying so much that it cracks and splits. Using foam instead of pywood would be a good idea.
- leaving some gaps on the straight track would be a great idea, also.
my thoughts, your choices