An interesting thread. I have often watched the action of the water on perfectly sheeting glass. I don't know if it's the surface tension, but there is a tidy thickness of water on there. You can easily overtake the water as it flows down when your rinsing.
In affect you are rinsing while dirt has not had a chance to run off from higher up the glass.
Try this experiment and you will be surprised.
Clean your van windscreen and it sheets perfectly, you think it's dry until you try and drive a few hundred yards without the wipers on, you will soon see just how much water is still on there.
That film of water on sheeting glass seems to come off in layers, the top layer comes off pretty quickly, but if you stand and watch it their is a slower moving layer just underneath it.
As for those spots at the top of the pain, I would have thought that Jeff using his over the top jets would be less prone to it, but the source of the spots has to be dirt from the top of the frame. When the jet of water hits the glass it spreads out in all directions, so when rinsing the top of the pane some of that water hits the frame. As we come down the glass we can angle the brush a bit more so the jet is facing slightly downwards pushing the dirt towards the bottom.
I have tried and still am using jets on stalks and this has made a difference.
With the jets being in contact with the glass there is less inclination for the water to bounce back, and onto the top of the frame.
I do a job once a month inside and out, I had a Bentley brush for the tops with jets on stalks, I used the Superlite brush with standard jets on my small pole for the bottoms.
The tops were almost perfect whilst some of the bottoms had spots near the top of the frame.
I think that the spots are caused by soluble dirts that contaminates the water, and as most sheeting glass will bead at the edges as it dries this is where it collects.
As the glass dries the beads get smaller and smaller, the surface tension dragging the dirt with them. Eventually the bead dries leaving a spot.
I'm no scientist, and I'm only relating my own observations. My theory could be entirely wrong