I suspect originally, when the rosettes were being machined into the reliefs, that the machining was slightly out of 'register' (Perhaps the wrong word) with the rest of the design at that point.
As soon as you said, "one pass" I then agreed with your suggestion.
I am only a US pretender, I know nothing of the production of their stamps, but using the British Rose engine as an example, I think the scroll work was produced vertically on a face plate in a Rose Engine lathe.
"Slightly out of register" makes perfect sense to me, this is very fine work.
Takes nothing away from the stamp, makes it more interesting.
The lathe work between the rosettes is just beautiful, beats the framework of the Penny Black in my opinion.