It looks like the same thing that happens on the SYD-JNB route with the SA codeshare flights.

The 'missing' seats are held back for the codeshare partner to assign. Usinf EF if you look at the F seat map on those flights you will also see missing seats. Then if you look at the codeshare (LA) numbered flights the missing seats are shown and the QF assigned ones are missing.

I could be wrong but I suspect this is the case here as well.