You my not have gotten far enough to check left arm/chainstay clearance, but that's one possibility.
Consider also that the total crank offset is based on the spindle length and the offset built into the cranks. If the right arm had more built in offset, the left side of the spindle would be longer to compensate, so pedal Q-factor was equal on both sides.
There could be other reasons including the overstock issue you mentioned (though I doubt it), but be aware that spindles longer on the left aren't all that rare.
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