Even with the openers tightly shut there is often a slight gap behind the opener frame, between the opener and the window frame. Any water above can seep into this gap, run down behind it and then emerge onto the frame of the main window below. As it runs down behind it can collect dirt from where the brush can't reach and carry it down on to the lower pane.
The only guaranteed way to avoid this would be to clean the 'hidden' parts of framework by hand with the opener open.
Even that won't last for ever as more dirt will accumulate and cause the problem to recur.
Some windows will be more problematical than others, as always. Brushing the opener frames with the water off will give a reasonable result on the strip between the openers, the rest of the window can then be done as normal, taking care to avoid the centre strip.