Here is how the PGA states the driving distance average is calculated: "The average number of yards per measured drive. These drives are measured on two holes per round. Care is taken to select two holes which face in opposite directions to counteract the effect of wind. Drives are measured to the point at which they come to rest regardless of whether they are in the fairway or not. (101)"
It's not stated here, but I don't believe Par 3's are ever used in the calculation. The numbers don't reflect the choice of club, so if someone plays it safe with a 3w or 2i from the tee that is being used calculate driving distance those numbers will skew the overall average. The fact they use only two drives per round that are equally weighted means that the averages are may also indicate aggressiveness on the tee. Golfers who tend to be more aggressive on a course and use driver more often will likely average higher than conservative golfers that put their 3w or 2i into play frequently to avoid trouble.
Originally Posted by Edgk
Full year Tour driving average is not a total of every drive hit. 2 holes are picked at random and players drives are measured on those holes and that is what becomes the driving average.