The panels themselves are usually made from aluminum U profile and close to the edge there is a substantial amount of trapped air if the panel is at a bad angle. Given that most panels are at a bad angle this will cause the top edge cells to all be over temp and since they're all in series that drops the efficiency of the whole panel. So the cross members certainly don't help but the panel construction itself could do with some more ventilation near the top. I wonder if cutting some slots in the top members would drop the cell temperature in a way that it would show up on a measurement, this is pretty easy to test.