Closed comb razors usually have different heads compared to open combs to give some space above and below the blade. With an open comb razor there won't usually be any space above or below the blade, apart from between the teeth.
Although open comb razors are thought to be more aggressive, the Merkur ones aren't, but opinion suggests the Muhle R41 is aggressive. If the comb was closed but with the same style head as an OC, the Murker razor would be very mild. So it could be that the balance has been achieved between a no gap closed comb mildness, and open comb aggression with the Merkur OCs. I find my Edwin Jagger razor more aggressive and that's a closed comb razor.