I'm trying to estimate a transfer function with given data (input and output). Could someone help me with this?

Oke so I used the systemIdentification tool of Matlab to estimate a transfer function for my centrifugal pump. I've got the characteristics of the model attached in the file. Here I would like to get a transfer function that gets an input (Torque x angular velocty) and delivers an output (pressure x flow).

As the input I used the power curve and as output I used the Q-h curve that you see in the first picture. For this model I estimated two transfer functions. One with zeros = 3 and poles = 4 (red) and one with zeros = 4 and poles = 5 (blue). As you can see both the transfer functions come close to my model, but the step responses look totally different. The one with 3 zeros and 4 poles even goes to a very large negative value, I don't understand this. Could someone explaine this? The transfer functions are given in the file as well.

Could someone as well tell me if I followed the right steps to determine the transfer function for my model?