The predictors predict the percentile/ranks dependent upon the past year trends provided to it. Now every year due to various parameters there are statistical fluctuations in the examination data. That the reason why such fluctuations are observed in the end results. But if you observe it gave you quite a precise value which not entirely absurd and such fluctuations are probable that's why we call it predictor.

I hope my answer helps. All the very best for your future endeavors!