in maths 2nd live test i got full marks . still my percentile was 97 out of387 .why

First of all you have to understand what a percentile is?

A Percentile is the value of a variable below which a certain percent of observations fall. For example, if your percentile is 97, it means the 97% of the total candidates who appeared in the test are below your performance.

It might be possible that some other students like you scored full marks, so they will be considered equivalent to you. It clearly means that the 97% of the total students are below you and the 3% are there who scored full marks. 

I hope now you will get this point about percentile!!