Steve,
I'm getting old so I may not explain this exactly right.
If I remember my fluid dynamics correctly, the mismatched ports create "eddies". These areas are stagnant and don't actually flow with the intake stream. They kind of create an invisible funnel effect at the oval port entrance.
As an example think of this crude comparison.
Which has less restriction?
1) a 24" piece of 3/8" id fuel line or
2) a 24" corrugated flex funnel(with 3/8" outlet)
The funnel will have a higher volumetric flow for the same value of negative pressure (vacuum), even with irregular inside surface (from the flex corrugations).
This is due to a function of laminar flow and length to ID ratios for the two flow paths
I hope this helps. Its 2 am and my brain hurts now.
PS: We used the following trick when we were to poor to afford rect port heads and wanted to run an aluminum holley intake.
We would epoxy about 2 inches of the rect. intake runners and port match them to the oval port heads. This created a smoother transition from the rect to oval runner.
This combination ran better than a full rect port setup a lower rpms, but didn't carry the power past 6500 on 396's.
That's all for tonight folks.
Eric