Yasmina
Yasmina,
Melbourne Open Days can be crowded, the one I went to had around 130 people. It's true that they make the OD into an AD if there aren't too many candidates. This happened in Adelaide where my friend was recruited, there were only around 70 people.
I think the email they send out is random and has nothing to do with if they think you're a good fit or not. The recruiters wouldn't even know who got those emails. And I got the email seconds after I submitted my application, so I doubt anyone had even looked at my profile.
I've never seen any AD's in Australia since I've been following their website (2 years). I think they only do these in cities where they get enormous number of candidates.
It would be easier to stand out where there are fewer candidates, but try not to think about this too much. At the end of the day, if you have what they're looking for and can deliver on the day, I don't think it matters where you go. Just as an example, when I reached the final interview, there were over 150 people at my OD. The second time I tried out in Zurich (I was traveling), they had less than 20 candidates and I didn't pass the first round.
Good luck!