Problem with scikit-learn metrics in K-fold cross validation

[BRN]

Hello everyone,
In my implementation of a K-Fold Cross Validation, I find a difference between the accuracy calculated during each training and the average accuracy calculated through the metric functions of Scikit-Learn.

This is my code for the K-Fold Cross Validation and for the calculation of metrics.

def kf_validation(images_path_list):
    
    kfold = KFold(n_splits = NUM_FOLDS, shuffle = True, random_state = 42)
    
    model = inceptionV3()
    
    acc_list = []
    mse_list = []
    mae_list = []
    auc_list = []
    
    for fold, (train_index, test_index) in enumerate(kfold.split(images_path_list)):
        
        print('==================================================================')
        print(f'-----------------------FOLD {fold + 1}, -------------------------')
        print('==================================================================')
        
        dataset = get_dataset(images_path_list, split_dataset = False)
        train_ds = dataset.skip(len(test_index)).batch(BATCH_SIZE)
        test_ds = dataset.skip(len(train_index)).take(len(test_index)).batch(BATCH_SIZE)
        
        model.fit(train_ds, epochs = NUM_EPOCHS, validation_data = test_ds, verbose = 1)
        
        y_true = [label for _, label in test_ds]
        y_true = merge_tensors(y_true)
        y_pred = model.predict(test_ds)
        y_pred = tf.argmax(y_pred, axis = 1)
        y_true = tf.argmax(y_true, axis = 1)       
        
        results = calc_metrics(y_true, y_pred)
        acc_list, mse_list, mae_list, auc_list = zip([(results[0], results[1], results[2], results[3]) for _ in range(4)])
    
    print('----------------AVERAGES METRICS AFTER ', NUM_FOLDS,' FOLDS---------------------')
    print(f'average ACC: {np.mean(acc_list):.3f}')
    print(f'average MSE: {np.mean(mse_list):.3f}')
    print(f'average MAE: {np.mean(mae_list):.3f}')
    print(f'average AUC: {np.mean(auc_list):.3f}')
    print('--------------------------------------------------------------------------------')

    return y_true, y_pred

def calc_metrics(y_true, y_pred):
    
    acc = accuracy_score(y_true, y_pred, normalize = True)
    mse = mean_squared_error(y_true, y_pred)
    mae = mean_absolute_error(y_true, y_pred)
    
    fpr, tpr, thresholds = roc_curve(y_true, y_pred)
    auc_val = auc(fpr, tpr)
    
    print(f'-- ACC={acc}, MSE={mse}, MAE={mae}, AUC={auc_val}, --')

    return acc, mse, mae, auc_val

For example, for the first fold, the result is this

==================================================================
-----------------------FOLD 1, -------------------------
==================================================================
Epoch 1/10
19/19 [==============================] - 55s 486ms/step - loss: 1.7101 - accuracy: 0.7681 - val_loss: 8.6331 - val_accuracy: 0.4371
Epoch 2/10
19/19 [==============================] - 9s 355ms/step - loss: 0.4822 - accuracy: 0.7729 - val_loss: 8.0062 - val_accuracy: 0.4780
Epoch 3/10
19/19 [==============================] - 9s 379ms/step - loss: 0.4996 - accuracy: 0.8312 - val_loss: 7.9097 - val_accuracy: 0.4843
Epoch 4/10
19/19 [==============================] - 9s 357ms/step - loss: 0.3882 - accuracy: 0.8738 - val_loss: 7.9579 - val_accuracy: 0.4811
Epoch 5/10
19/19 [==============================] - 9s 355ms/step - loss: 0.6962 - accuracy: 0.9085 - val_loss: 5.0147 - val_accuracy: 0.5283
Epoch 6/10
19/19 [==============================] - 9s 359ms/step - loss: 1.1334 - accuracy: 0.8896 - val_loss: 2.6646 - val_accuracy: 0.7453
Epoch 7/10
19/19 [==============================] - 9s 349ms/step - loss: 0.3623 - accuracy: 0.9401 - val_loss: 4.8177 - val_accuracy: 0.6792
Epoch 8/10
19/19 [==============================] - 9s 354ms/step - loss: 0.3827 - accuracy: 0.9401 - val_loss: 3.4249 - val_accuracy: 0.7767
Epoch 9/10
19/19 [==============================] - 8s 347ms/step - loss: 0.3808 - accuracy: 0.9180 - val_loss: 4.9683 - val_accuracy: 0.7107
Epoch 10/10
19/19 [==============================] - 9s 361ms/step - loss: 0.3999 - accuracy: 0.9148 - val_loss: 9.7832 - val_accuracy: 0.3396
10/10 [==============================] - 4s 68ms/step
-- ACC=0.5125786163522013, MSE=0.48742138364779874, MAE=0.48742138364779874, AUC=0.5153103611979271,

How is it possible to obtain an average accuracy of 0.51 when in the training is in the range 0.70 - 0.90?

Does anyone have an explanation?

Thanks.

 

[BRN]

Maybe I explained myself wrong. I wonder why the accuracy calculated with the Scikit-Learn metrics is not comparable with the one displayed during training.