Leave a thin air gap and each surface can have its own curve. Fraunhofer used that extra freedom in early-nineteenth-century objectives, along with better glass than Dollond had. The pair is no longer one glued pill like the cemented achromat.
Cemented is sturdier; air-spaced is slower to make and easier to get wrong. On the board the gap is the whole difference — same two glasses, one more radius to play with.