The way parallelism is calculated matters which plane is first. In Measure, all planes are mathematically infinite, but when calculating parallelism, constraints on the size of one of the planes must be used which results in this behavior.
The datum plane specified is an infinite plane. The 2nd plane, is not infinite during the parallelism calculation, its size or end points are determined by where the probe actually touched. A perpendicular line is constructed from these end points to the datum plane. The distance between the length of these lines along the normal vector of the datum plane is the parallelism value. If the angle between the datum plane and the 2nd plane does not a change, a plane that is longer will have a higher parallelism value than if only a shorter portion of the plane was measured.
To avoid this, you can key in the length of the 2nd plane on the parallelism screen. For example, if a plate is 5 inches long, you can key in 5 inches. This will set the end points to the predefined value which can give more repeatable results.
parallelism, repeatability,2 planes, two planes, 2 lines, two lines