Why does training score start from 1 when using sklearn's learning curve plotting?

Question

I have a question regarding sklearn's learning curve module. I have been trying to use it to plot the learning curve for two different models and but for some reason which I cannot understand, training set curve starts from 1 and not from 0. Validation set curve starts from 0 as far as I can understand even though it never really apears to be starting from 0. Can anyone help me understand what's going on?

sample learning curve

train_sizes, train_scores, test_scores = learning_curve(
    clf_pipe,
    X=X,
    y=y,
    cv=5,
    scoring='balanced_accuracy',
    n_jobs=-1,
    train_sizes = np.linspace(0.00001,1,20),
    shuffle=True
)

In this case, clf_pipe is a pipeline of a standard scalar method and logistic regression.