Not a glazier but my Guess would be that the area of the glass where the beads are accumulating is of a higher surface roughness than where no beading occurs, this increased surface roughness, causes greater forces of attraction at the interface between the glass & the water, so the water stops there instead of rolling down to the bottom of the pane & onto the sill. The fact that the pane is so large may be a factor in maintaining consistent surface quality across its span during manufacture. Try viewing the area from different angles while placing a white sheet on the inside to see if any imperfections are noticeable.
Like i said I'm not a glazier...speak to one.