A bulge can be caused by overpressure. A concave top can happen by vacuum in the tank, and I have seen tanks from HD, which were stacked 5 pallets high, with the nipples crooked because the top of the tank collapsed some. This happens on the bottom of the pallet. They are shipped just one pallet high on the truck, so this only happens in the receiving area of the big stores.
I assume you did NOT get that guardian model at a box store.
Hard to explain why you have one tank one way, another opposite. It is very unlikely to be a manufacturer defect, so Rheem will dodge this one.