The dotted ones are the actual measured curves and the normal lines are the fitted curves. The measured interaction is not 1:1 and therefore the fitting tries to compensate with large bulk jumps in the fitting.

Moustafa, hard to judge since the instrument type used is unknown. The solid lines seem to be the recorded data (including huge buffer shifts), the dotted lines seem to show fitting trials that ignore the buffer shifts. - You should ask the person (from whom you received the data) for details about data recording and fitting parameters.

Hans-H. Trutnau Thank you very much for your answer. The instrument is Biacore T200 with CM5 chips at 25 °C.