Yeah measuring in diameters is pretty strange but it was the first thing that poped into my head that would yield factors of pi.

Could also be that they a standard unit (1u) and would use both 1u radius wheels and 1u circumference wheels.

Or its all derived measures like "the length of a wheel spoke" and "the length of a string around the same wheel" so never the actually physical wheel is used for the measuring.

Maybe we should ask the tired archeologists?