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Just got off of the phone with a metallurgist/mechanical engineer. Here's what he said:
When you are breaking it in you are setting the right balance of hardness on the outer surface of the gear. There is no real geometry changing.
When the gears are manufactured there's a hobb setting which defines the exact environment that they were manufactured under. This is what is inscribed on the gear in tenths of thousands. At GM they probably have this setting and can spec the rear setup to minimize the need for a break in. In a shop you don't have their type of setup and a break in is needed to make up for the lack of industrial environment. Street gears for non high performance cars are harder than high performance gears and aren't as sensitive to the lack of a break in period.
He mentioned the cross sections of the gears and their associated Rockwell hardness however I do not recall enough of that to post it completely. Something along the lines of the whole gear set being hard enough but the alignment of molecules being key for wear protection.
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