The time behind (or in front) of a rider seems to be calculated by the distance divided by an average speed. This is not the real time difference. When a rider in front starts to go uphill, you gain fast on him/her. When he has passed the top he gains quickly on you. It is not really possible to see if you are actually gaining (or loosing) until after one lap you pass the samen position and remembered the time from last round.
It would be nice if this could be done more sophisticated (like you see on tv). Easiest would be to have a time grid based on an average speed where the grid space is smaller on the climbs and larger on the descents.
More sophisticated would be to remember the time the rider passes a point and then compare that time with other riders on the leaderboard passing that position.