The calculation of mean planes in Dips is a vector mean. This is not equivalent to a simple numerical mean calculated by averaging Dip and Dip Direction values.
For example, say you have two readings of Dip Direction: 5 degrees and 355 degrees. The true (vector) mean is 0 degrees, whereas the numerical average is 180 degrees.
The Global Mean Vector is equivalent to including ALL of the poles in a single set, and calculating the vector mean (same calculation as for a Set).