I think Its just a normal practice among us. We always refer to the size of the hex head whether its bolt or nuts. It follows that these sizes help us to choose suitable wrench or socket to be used. Whereas when ordering a replacement - let say to replace a rounded head fastener - the bolts and and nuts sizes are specified according to its thread width. For example, I've ordered copper-flashed replacement exhaust nuts for the car with thread size 8mm 1.25mm pitch at eBay. The exhaust studs are 8mm width. However the order arrived with 12mm hex head, not 13mm as per original Volvo.
Amarin.
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