I guess they follow some risk profiles and categorize applicants. This is only my assumption. I do not know how they do it but i assume, for example, i am someone who do not have a travel history or have not claimed points for work experience. My application is for single applicant. Hence, my case should fall under low risk. The risk also takes into consideration the country from which you are applying. If you take my profile as a baseline then i guess we can judge how the department would handle when each of these factors come into picture.
But yeah this could be entirely wrong and only plausible explanation could be "DIBP only knows"