Here's what I think is happening. Each element of a row appears to be typeset on the same baseline.
It looks as if the baselines of adjacent rows are separated by the maximum of the separations that would be needed in each column. This assures that no entries collide with each other--the tightest packing consistent with the notion of baseline. I think what most of us expect is that the separation should be the maximum excursion below baseline in the upper row plus the maximum excursion above baseline of the lower row plus a standard gap. This assures that there is a horizontal band of white space between adjacent rows. I haven't experimented to see whether the ultimate baseline separations are equalized throughout the matrix or vary by row. Doug
