30mm vs 30.4mm Wheels & Tires

[tom alphin]

I think one reason might come from how we choose to describe these.

All ldraw description should be the same. Rim (width) x (diameter) in millimeter. The rim and the tyre share a common size, here it is "30". 
They belong together.
If I check all of these in ldu they all measure 76 ldu, which translate to 30.4 mm
And then we round it of to the nearest integer. No decimals,  30mm

That's good enough confirmation for me, thanks.