I'm guessing that you're optimizing things by raycasting against each surface, in order, and if theres a hit it skips the remaining surfaces (even if they're above the current one). Whatever the cast, in many circumstances all I (and possibly others) want is to have the instances placed on the highest surface, so that if some structure is sitting on top of terrain, the items should be on that rather than the terrain below. Perhaps there could be an option/toggle to place on the highest point, rather than on the first ray hit.
I've attached an image to demonstrate.