Hi Nick - if you turn on the surface control points it should be clearer what is happening.

MoI's Planar command builds an underlying surface that is somewhat extended larger than the boundary edges.

There is a command in Rhino called "ShrinkTrimmedSrf" which you can use if you want to shrink the underlying surface down close to the boundary's edges - if you run that on the plane that has different UVs, then it should make it the same as the others.

MoI also has a ShrinkTrimmedSrf command as well, but to activate it you have to set up a keyboard shortcut for it, there isn't currently a button for it.

The other methods that you show there like Loft and Sweep don't happen to make a surface that is larger than the trim edges.

- Michael