@shilpa6 The person who added the problem assigns the difficulty based on his/her own judgment. Therefore, the level of difficulty set might be subjective. We regularly look at forum posts to see if people think it is too easy/hard, and readjust the difficulty level.
Easy - Median time to solve within 15 minutes.
Medium - Median time to solve < 40 minutes.
Hard - Median time to solve > 40 minutes.