When dimensioning between 2 planes or 2 lines, the minimum distance between 2 features is reported. The length given is measured from the center point of one feature to the perpendicular distance of the other feature. If both features are not perfectly flat or repeatable in form, you will get varied results every time you measure the 2 planes or 2 lines.
Because it is very rare you will run into 2 perfect planes, a better method would be to use a point command (like Comp Axis or Surface Point) and dimension a point to the corresponding plane or line. The numbers will prove to be more repeatable.
2 planes, 2 lines, plane plane, line line, line dimension, plane dimension