ScikitLearn - RandomForestRegressor score different in and out of grid search
I have found the answer to my question : there is randomness in RandomForestRegressor().

Using the random_state hyper parameter fix the problem :

rfr = RandomForestRegressor( random_state = 123 )


What is surprising is the difference of score obtained from different values of the random_state hyper parameter. It means my best hyper parameters n_estimators and min_samples_split are strongly depend on the random_state hyper parameter. 

I guess it is something I will have to deal with.