# isolation forest erratic behaviour

I have gotten a dataset that has continuous values which are normalized in the range from 0 to 1, is it approximately 200 records with 14 features, and like 30 percent of the data present there are outliers. I have tried to use the Isolation model of Scikit, and now I am performing the hypeparameters tuning. So far what I have done is the following:

parameters = {'n_estimators':[10,20,40,60,80,100], 'max_features':[0.1, 0.2, 0.3,0.4,0.5,0.6,0.7,0.8,0.9,1.0], 'max_samples':[10,20,30],'bootstrap':[True,False]}
clf = GridSearchCV(iso, parameters,  scoring=scoring,n_jobs=-1,verbose=5,cv=10)


and for the scoring function I have used a weighter F1-score. The problem is when I get the best hyperparameters results, for example:

best parameters  {'bootstrap': False, 'max_features': 0.3, 'max_samples': 10, 'n_estimators': 40}


and I apply these values I got some erratic behaviour everytime that I run my model to obtain the ROC curve. For example in one run I can get:

but when I run again my model with the same hyperparameters I can get something like this:

why this could be happening?

Thanks