First of all, let me tell you how a rank predictor works. The coaching institutes generally get the scores input from the students, so that they have a database ready for analysis. They then consider all the options like difficulty quotient, that compared to the previous years, their enrolled students’ records within the institute through the couple of years, statistics obtained from other classes and finally simulate a ‘Rank Predictor’.
Every institution releases its own answer keys and based on that your marks varies from institution to institution. So it naturally becomes difficult which score to consider, as each would eventually lead to a significant difference in your possible-ranks, even on the same predictor.
All the best!!