Intro to a port job

Why does THIS graph of lift progress in gradual increments of flow increase as the valve opens (which we would expect) and some lift flow numbers have near matching flow for .300 AND .400 with a tremendous taper off (falling flat in flow increase) between .400 and .500 (or something similar)?
THANKS for teaching us!!!
Great question, one I would have asked way back when but would safely assume now... well, Because I didnt really finish the short turn for this valve and turbulence takes over pulling the air to one side and no longer flowing more air. Specifically the straight side short. I ground down the wall hump and went into the straight wall in the process... but then you need to position high in through the port window and grind down toward the valve/bowl and make the shape that completes the other dog leg side short. Maybe I'll continue, but I'll have to do it all over again when the 2.02 is in like I said in that post you quoted. We could see much more if this was the finish valve size.
As for the part about near matching .300 and .400 numbers? Are you referring to the 2.02 numbers posted from the other heads or?
The increase over stock 'with same valve' flow is...
.100 +15
.200 +14
.300 +20
.400 +21
.500 +54
These 348915 heads with the work done to this point ,as is, are better than those 2.02 308 with the 'suck ***' valve job we started out with even after porting. Sunk is junk.