Looking at your graph I see the little dip in the 50 that starts about .250 lift. It’s more pronounced with a 55.
I tried for years to understand it, but the last I recall about what we came up with was it was a l/d verses the widths of the cuts under the seat.
I know we found a couple of valve jobs from other people that had narrower cuts under the seat and the dip would go away.
The problem was it would flow more air with the narrow cuts but it always lost power.
I guesstimated that with the narrower cuts the valve job was functioning as a radius and that killed power.
It is there on Chrysler heads, Chevy heads and even a 351 Cleveland I didn’t port but flowed. Some of the Chevy testing was done on AFR 227 raised port heads.
To this day I’ve never seen an explanation for that dip. Or why getting it out of the flow curve hurt power other than my WAG on the valve job acting like a radius.
Edit: bad typing. I meant to say where the curve goes through .250 lift.