13/21 as a percentage is 61.90%. This is the rounded value you get when you change the fraction into a percent with two decimal places.
The fraction 13/21 is a proper fraction, meaning the numerator is smaller than the denominator. It is a little more than half, so the percentage being a little above 50% makes sense.
People often use this kind of conversion when they need to compare parts of a whole in everyday situations, like scoring or budgeting. Remember that the 61.90% figure is rounded, so the exact value is a tiny bit different.
