My washing machine detects how much stuff is on the various racks and will adjust its time running the sprayers for each rack based on how much stuff is there and the turbidity of the water (how dirty) it is for the given levels.

So if the top rack is pretty empty or just glasses which are mostly clean, while the bottom rack is full of pots with lots of grease, it will spend more time operating the sprayers on the bottom rack than the top rack.