SKLEARN GridSearchCV hinting higher accuracy than Pipeline but with same parameters as Pipeline estimators

I have pipeline estimators like this:

text_clf = Pipeline([
      ('tfidf', TfidfVectorizer(max_df=0.95, min_df=2, max_features=n_features, stop_words='english')),
  ('clf', SGDClassifier(loss='hinge', penalty='l2', alpha=1e-3, random_state=random_state, max_iter=5, tol=None)),
])
text_clf.fit(dataset.data, dataset.target)

Then when evaluating the model accuracy like this

mean = np.mean(predicted == twenty_test.target)
print("mean %0.3f" % mean)

I get score 0.802.

Then when I add the GridSearch to get the best params like this:

parameters = {
     'tfidf__use_idf': (True, False),
     'clf__alpha': (1e-2, 1e-3),
}

gs_clf = GridSearchCV(text_clf, parameters, cv=5, n_jobs=-1)
gs_clf = gs_clf.fit(dataset.data, dataset.target)

(Note that i am fitting GridSearch on TRAIN data same as the pipeline – although when i first tried fit it on TEST data by mistake the result was same though.)

It reports this:

grid search best score 0.868
Best params: clf__alpha: 0.001
Best params: tfidf__use_idf: True

Note, that these params are already set on the model but the score is lower in the pipeline.

Is it because the other parameters I set in Pipeline are not kept when using GridSearch or?

Btw how the Grid Search knows the best parameters if I didn’t provide any testing data.

Another problem is, that when I added more parameters to adjust in Grid search and then applied it to Pipeline, the accuracy didn’t change at all. (or changed from 0.802 to 0.805, but GS hinted 0.867)