Here's my theory for the chart:
Their tracking system tracks ALL of the shots they take. Now remember, scoring clubs for them are 8i-SW. These are clubs that they can easily control the distance with. So let's take 4 typical shots with a PW for example:
135 yds - very smooth, controlled swing
120 yds - 3/4 swing for accuracy and less spin
150 yds - full swing
140 yds - smooth, controlled swing
Average - 136 yards
Even though their full swing results in a 150 yard shot, they rarely hit that sort of swing UNLESS they have a perfect lie, such as off a tee box; I see many pros hitting a PW from 150 off the tee with no wind or elevation. That's why the averages are that low.
I think the pros can hit a much longer than a 135 yard very smooth controlled swing.