IIM Rohtak offers admission based on your potential and not solely on the marks previously obtained. When the interviewer enquires of you regarding your low percentage, you should honestly answer him/her.

Instead of faltering and mumbling your answers you should confidently speak out your answers. Even if you would be at fault your interviewer would let it slide given that you are not doubtful of your own self. Try not to use any falsehood around them and notably maintain your calm. 

One can be of the view that your personality impresses your interviewers. In some cases, this view could be true. Maintain your calm and confidence in yourself and you will crack the interview.