Well, based on my experience, here's what I think. CX doesn't want to sell any more codeshares than it has to, because it doesn't want to share the revenue with anyone else. That's why CX shows A4, and the AA codeshare shows A1 on the same flight. This is very common with CX. You'll usually see a similar difference in F.

The AA RTW desk will try to sell the codeshare whenever it can, even when you don't request it, because AA also wants to maximize revenue. However, if it's only available as a CX flight, the AA RTW desk will still give it to you. This has happened to me, and I'm almost always looking for only one seat.