It's the same way with a Bulli, and I suspect it has something to do with how the threads on the bases are made, no two seem to be exactly alike. The air hole has to be drilled so it is precisely to the side of, and between, the two screws for optimum performance.
The cone must be placed onto the base piece and marked where the hole should be placed to ensure it's in the right place. With a bulli, you have to drill the hole yourself on the 'downgrade' tube for this reason, or order it at the same time as your tank kit and inform the maker that you want to the two to match so he'll drill the hole for you.
If every piece was exactly the same, this wouldn't be a necessity, but that's not the case; I prefer to think of it as 'old world' precision craftsmanship.
Lisa my lovely, how are you doing this fine morning?