They're included in the rankings because every year the number of students that drop out change, so they need to do it to ensure that ATARs remain comparable across years. Also the drop out rates are different in every state, so it has to be done to compare interstate ranks fairly. The reason they choose year 7 is that it's compulsory in all states and very, very few people leave school before year 9.
It doesn't affect anything really as the ATAR is a rank, and you'll still maintain your rank relative to other year 12s no matter what system they use for comparison.