Good point
I see it as being similar to a snowdrift. I've had cases where I've had snow drifts over a foot deep in part of my lawn and no snow at all in other places. The places where a parachute would be most likely to end up would also seem likely to be where you'd find "dust drifts." While the overall dust accumulation could be very small over a wide area, it seems possible that drifts could be much deeper near rocks and similar features.