If I understand your question correctly, you want to know why the fit is better for simulated data against the experimentally obtained real data. If so is the case, in my opinion, it has to do with the order of the model. You see, the simulated data is obtained from a model whose order is exactly known. However, in case of fitting a curve on data obtained through experiment, the model is an "approximation" of the real system. The real system may be simpler or many times complicated or require additional parameters like delay etc.

Secondly, the experimentally obtained data could be too noisy with some datapoints deviating too much from the steady-state value.