I would guess that is because the heads exhaust port may not support airflow above a certain lift, but it can be compensated for by keeping the valve open longer. Thats why it is so important to match the cam to the heads and not vice-versa, yet people still often insist on picking off the shelf cams before they even know what heads they will be running .

I would not second guess any competent builder/machinist/cam guru with a good reputation . What they do is not something the average person can learn from a book over a weekend - years of experience and trial and error are behind their decisions, not just typing specs into a computer.

 

In my case, I bought the heads long before I bought the cam. I think my exhaust port flow #'s are exceptionally good, see flow #'s above. He could have picked other exh. lobes with less duration, like the 244°/.613 lift Grind # 3120. The thing is, I ordered a custom cam, which later on it turned out to be an off shelf Comp Cam!!!!
Again, I think I don't have the right cam since it is not a custom cam specifically picked out for my combo.