Probably because factories use what they have in front of them in their trays. When one batch of parts run out, they start on the next, and there is no guarantee that they are the same, or better (or worse for that matter).
Also... For what it's worth, 20mm springbars will have a total measurement of 22mm to 23mm end to end, as the ends must extend a bit further out to engage in the lug holes. Perhaps some factories don't pay attention to such measurements, or perhaps they use a lesser product (i.e. 21.5mm in length, weak spring, etc).
Fact of the matter is... Sometimes the QC isn't what we'd like it to be...