Regardless of how much you overtap, the molten Zinc base metal used in the Galvanising process fills and clogs the screw threads making it difficult if not impossible for the nut to engage correctly with the male fastener. Consequently a tapping operation needs to be carried out (usually oversize) to clear the threads sufficiently.
Sheradising, Electroplating, Electro-Galvanizing and PTFE coatings, on the other hand, are far more consistent in thickness than HDG. It is common practice for the tapping operation to be carried out before the coating operation in these instances.