This is a bit of a sticky situation in that the manufactures of pneumatic or mechanically fastened nails exploited what had always been the traditional sizing for nails.
Back in the day nails were sold by penny weight I.E. the weight of a nail weighed per 100 nails to determine the size ie 6 penny, 8 penny etc. Too compete with the less expensive hand driven nails the mechanically driven manufactures added the weight of their nail bundling weather it be plastic as with a stick nail, or wire with a coil nail into the weight of 100 nails to quantify the classification.
The issue was addressed in the FBC 2001 with exacting specifications as too size of shank, length, and the size of the nail head, but only the savoy building inspectors, designers, and contractors picked up on this change.
The reality of the matter was the contractor ordered 8 penny nails the box said 8 penny nails but in reality they did not meet the sizing requirements and it was not until the 2004 Building Code went into affect in October of 2006 that the building designers, contractors, and manufactures were all on the same page.
I’m not sure the designers still trust the system in that they generally require a 10d nail to error on the side of caution.
So likely if the roof is nailed at 6x6 the contractor probably thought he had done the correct job; after all the box of nail that he bought were labeled as 8 penny