Biggish sky theory at work
CTAF radio procedures have always been a ugly compromise anyway - unless operating/monitoring with 2 radios - one on area and one on CTAF.
1 aircraft just inside CTAF on CTAF freq & 1 aircraft just outside CTAF on area can be broadcasting correctly just not talking to each other and heading straight for each other.
Jandakot- Rottnest is a classic. Luckily Perth Centre has radar to advise the area frequency aircraft of the other aircraft. And Rottnest is marked on maps...