I guesstimated what a larger 2.14" valve's average curtain diameter would be and plugged it in but it only had a slight changed in the square foot curtain area was 0.00157 (from what I guesstimated) vs 2.055" 0.00151.
Had a pretty good feeling this was why but now I get it for sure even though a 2.055" vs 2.14" seems like a big change to us, at low lifts the curtain area has only slightly changed.
Without drawing it I wouldn't know for sure, but with some quick math I believe you are thinking correctly. I'll try to draw a few more valve and seat combinations some time. It helps me see things better.
Now think about this. It is common for the discharge coefficient (efficiency of the venturi formed by the valve and seat) to increase from 0.050" to about 0.150" lift, then rapidly decrease after that. The decrease is easily explained by the loss of a good venturi shape as the valve opens, and by the rest of the port starting to affect flow. But why the increase from 0.050" to 0.150" (or maybe 0.200")?
This increase in Cd (or DC as sometimes noted) does mean that the average velocity through the venturi at the valve will exceed 350 fps when the Cd is highest.