Different results when evaluating as Plane vs Freeform

[[Jo...]]

I am a bit new to freeform. (read the book, havent taken the training).   

When evaluating a plane vs. a Freeform Surface with the exact same plane points recalled.  I am getting different values.  the difference is small but important in my measurement.  I am having a hard time wrapping my head around why they would show different FORM, sigma,  and min/max point.

What about freeform seeing a flat surface makes it calculate differently than the plane feature calculating a plane? 

Why are sigma values different?  Is one giving me the "sigma of a plane" and the other giving me the "sigma of an arbitrary point cloud"?

[[Ri...]]

Planes can be filtered and efficiently outlier eliminated, Free-forms cannot. I suspect that is the difference you are seeing. 

You can test this by looking at the flatness with both the filter and outlier elimination turned off.