I don't know what it is and maybe some other players can speak to their results, but I feel pretty safe saying that something is wrong with the Tour Starter and Ping 7-irons in regards to consistent distance.
I understand there are variants in length to a degree, but the 7I often tends to supercede the maximum allowed and hands down wins the most inconsistent iron award.
The Tour Starter 7I is rated for 142 yards and the PING 7I is rated for 145 and yet, on probably 90% of approaches, both versions typically only carry 128-132 yards. That is the same carry as the Starter 7I. Roll will typically add only 2 or 3 yards depending on where you landed, so the club almost never reaches it's full potential. I can think of only a handful of shots where the 7I played a normal distance without aid of wind or severe downhill and I can't think of any incidents where it went farther than it was supposed to.
Case in point is today. I had 140 yards left with a 10mph wind blowing towards 1 o'clock (tailwind from the players left). Knowing about the 7I's issues I ripped it for the full 142 (TS). Of course once again, 130yd carry with a 2 yard roll. It came up 8 yards short of the flag and 10 yards short of what I hit it for with the aid of a 10 mph tailwind. That just doesn't add up.
All other clubs will vary between slightly short and slightly long from shot to shot. The 7I however will only vary between way short and barely short. It's as if the distance variables were not changed when the new clubs were introduced.
Thoughts from other players?