You can't say this is astrology. Analysis can't be correct if full data is not provided. Nobody knows that how many cancellations would be done? How many other Quota seats would be released at the time of Chart Preparation? How many Extra Coaches would be attached?
The predictions are based on experience and historical clearances of WL# as experienced (not calculated) by members. I would suggest see the accuracy rates of members and go by them. If It would have been a totally astrology thing than you would seldom find accuracy rates more than 60%.