Can you show us this example because roundness or circularity or even cylindricity is a type of form to itself with no particular place to be, acts like flatness except on a curved surface.
Max and Min diameter is 2x the Maximum or Minimum polar radius of the Max/Min hits in a circle from the calculated centre.
Roundness is the overall form of the circle - so how big would the gap be between two concentric circle which contain all the points (these circles wouldn't have the same centre as the calculated centre as measured) - but as we're looking at the gap between them we're looking at it in a radial difference whilst the Max/Min evaluation is converting it to a diametrical difference.
Even so due to the difference in calculated centre vs the roundness 'centre' it will likely not be exactly 2x.
If DCC circle is using "minsep", the results must be exactly the same. If using least square, they can be different, depending also of the number of hits and the filtering.
