the problem with this site is that it gives an estimation of fee based on previous block instead of doing it based on memory pool. it is somewhat like trying to predict the weather based on yesterday's weather instead of analyzing the sky, clouds, wind,... right now.
so usually the fees they suggest is close to reality but there are always times when it is either higher or lower if the mempool had grown or shrank ever since last block was found. not to mention that last block's transactions might have been overpaying or underpaying.