Hi Burr, well a Coons patch is fundamentally a 4-sided thing.

When you do it with 3 curves there is an implicit fourth curve involved which is taken as a curve with all of its points collapsed down on to the same spot.

The location of that singularity point will have an effect on the shaping of the generated surface. Just turn on control points of the generated surface and you'll see how the points collapse down to a single point on one side, that's the difference.

It seems like the case you have here, your path curve is actually an exact 90 degree arc, with radius = 10.15, is that correct?

In that case it may better for you to use Revolve, just selecting one of those pieces as the revolve profile. That's usually the most efficient and accurate way to construct something that pivots on a center point and sweeps out a path of an arc.

- Michael