Incorrect Motor Speed Reading Using ISR Capture Module—How to Improve Accuracy?

I had used_ ISR_ method to capture the speed of motor. But it reeds the speed incorrect. I verified my code by capturing 50Hz of frequency from other programed controller .It reeds that 49Hz but why It fails to capture the speed of motor.How can i over come this problem Kindly suggest me to troubleshoot.... Thanks in Advanced to help.