PNR predictions are made on the following basis:
..
1. PNR History of the train
2. Quota
3. Class/Date of Travel
4....
more... Logic
5. # of WL: If date is far (3-4 months away) then Chances of CNF More
6. Some news about train, like augmentation with Extra Coaches. In such cases, WL will surely confirm if it doesn't confirm in normal cases.
7. Usually, people will mark CNF as a safe precaution. As the date of travel nears and based on status before charting, it is changed.