Hello Sankalp,
Percentile score means the percentage of candidates who took part in the exam and secured lesser marks than the candidate in the exam. The percentile score naturally changes every year as the number of candidates changes every year. So, as for your question, yes the 20% percentile for HSC will change and should increase in all probability as the number of candidates increases every year.
Regular exam updates, QnA, Predictors, College Applications & E-books now on your Mobile