I am not an engineer, but I think it has something to do with "beams" for how they work for strength. Panels flex and need to be held in place.
The "beams" are the panel frames which have the bottom of the L as the strong point ( I think--not an engineer) so that is where the holes are for the supports. The holes are placed along it to best advantage for distance apart and strength. The panel instructions warn against drilling more holes in the frame.
IMO the correct method is not to use Z brackets at all, but to use two- piece L brackets using the underside frame holes but still be able to remove the panel without yanking the bottom brackets out of the roof. IAW the side mounting is within the two-piece brackets not the panel sides.
