The estimation is based on the statistics for how much similar players were bought in the recent days. There is a rush to buy a player before play-off, and at the end/begining of the season, so the prices are usually higher at that time frame and lower during the season.
It is also important how old is your player. Younger players with the same TSP are more expensive thab older ones. Even if it was just one season your player and similar ones are not so atractive to buyer as they were one season ago.
I hope that explains your problem.