Pretty common with all tires, espicially offroad tires. The only possible explanation I could ever come up with was that they make the molds to 38", but due to manufacturing process, when they pull the tire from the mold they come out smaller.
The bigger question is why are metric tire sizes so gay?
If, when you were shopping for the tires, had used a metric conversion calculator (online - most tire manufacturers websites have them) it would give you the actual size. It is common for the Metric tires to be smaller, then depending on what wheel width you install it on it can be altered even more.
It's something that most 4x4 enthusiasts already know and deal with.
Did you ever watch a dragster do a burnout ? The tires get taller from the centrifugal forces of the tire spinning . Same idea for off road tires . If your axle gets hung up . Then you spin the tires as fast as you can . Your tires will get 38 inches tall and your truck will lift up over the rocks
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