First, of all no tool can tell your actual rank before the results, what Careers360 JEE Main Rank Predictor doing is we are taking past data into consideration and based on different statistical and prediction algorithms they are predicting the ranks, which can be said is close to the actual ranks.

Even full analysis was done, where the Rank predictor tools present in the market for JEE Main 2016 was compared with the competitors and actual results of the students, with proper analysis it came out that Careers360 JEE Main Rank Predictor was 75% in range of actual ranks .

Summary of JEE Main Prediction by Resonance tool as compared to actual JEE Main result:

JEE Main Rank Prediction as compared to actual result

Careers 360- True- 76, False-24
Resonance-True-44, False-56

You can read complete analysis of Careers360 and other competitors rank predictors-

CAreers360 Rank Predictor Outshines Competitors