Differences in Spreading Resistance. is the explanation I use for much of the behavior shown in the plot.. In its simplest form, bp thickness and area.
My many plots of C/W v "h" (related* to Velocity) and C/W v PD illustrate my interpretation of Spreading Resistance influence
E.g..
I,also, have no formal training in Fluid Dynamics.
* Usually expressed as a power relationship (e.g Sieder Tate) . In the impingement case I still like to heretically imagine it "proportional to Thrust(massflow rate x vel)