Percentile refers to your relative performance with respect to the other candidates. X percentile means that you scored better than X% of the total candidates.

On the other hand, XAT percentage is like the normalised score which you got in the exam. So if in Decision Making section, your percentage is 16.524 and percentile is 49.407, it means that your normalised score is 16.524 and you did better than 49.407% of the appearing candidates.

A total of 98 percentile means that your total score was better than 98% of the students. Even with 49 percentile in DM, you got a very good overall percentile. The reason behind this would be because your percentile in the other two sections- Quantitative ability and Verbal and logical ability must be really high. This would be because of high percentages in those sections, resulting in a better total percentage and hence a better total percentile. Hope it helps.