Because they are saying the spring can be OPENED to that lift with there spring... Before coil bind. they dont know what kind of cam your using, you have to plan ahead on that!
My old cam was no taxi cab cam though.It was 234 /244 duration @50 and it was a hydraulic with zero lash (plus a 1/2 turn im my case).
The solid is 254 /254 @50 with a hot lash of .028/.028.
Now subtract the .028 lash from the solid and your left with .480 lift on both sides (which is .030 less on the Exhaust than my old cam ).Wouldnt the lash of the solid also reduce the duration sum to over the hydraulic ?Im not arguing here im just trying to understand how these things are determined by cam manufacturers is all .I have researched the subject sum online ,but cant find the anwers .I doesnt really matter much as im doing what Isky recomends for the springs and installed height ,but still curious what its based on ...