I asked my teacher yesterday, his reply was that with AMEB, they have a person specialising in that (whichever instrument) to examine you. They do not travel around the world. =(
Whereas with other systems (what-not) (something from England, I forget the name), they have someone who conducts (or some other broad-ranged talent thingee person) who has attended many seminars, so they what to look for too with different instruments. These people travel around the world - very expensive indeed. This helps keep standards the same.
Examiners from AMEB also attend many seminars, to keep it fair within instruments (bad grammar, I know, but I can't be pooped fixing it). eg. It wouldn't be fair for people who did guitar exams, came out and all got A+, while everyone doing piano only got a B... =)