It's cause they took their exsting backplates and just stuck a block ontop of it, that's why it looks so random.
I suspect that they suspect the 3090's rear memory could be a one off, so they don't want to spend too much engineering resources making a fancy full cover block that next year will become end of life never to be seen again.
But if Nvidia continues to stick memory modules on the back of the card and they run hot, then I could see EK making a proper looking rear block
I own the original non active plate and the new active one. The active one isn't the original with a block bolted on to it. The original was entirely aluminium while the active section of the new backplate is copper and whatever upper you decide to go with. Around the block section where heat transfer is less critical and where the metal doesn't contact the liquid in the loop the metal is aluminium.
It's more like they took the original backplate, then cut away the centre and inserted a copper block that fits flush with the underside and leaves it looking like it is all one piece when it is not.
Regarding the way it looks, I agree it does look odd. However many x16 slots with a direct connection to the CPU are in the top slot on ATX boards. This is especially true for matx and itx.
I suspect that making the block small was intentional and a major design goal to make it work with as many motherboards and cpu coolers as possible for those that use the top slot.