Why is an Artificial Neural Network with high accuracy score giving poor results?
You're testing your neural net with data from the training set, of course it will get you great results. But you're neural net is now trained for a specific data and is unable to make generalization. This is known as Overfitting.

You should either split your data into test/train or use another technique to estimate your NN performance. A good technique is K-fold Cross validation.

Also check Overfitting wiki page and Cross validation on sklearn if you want more details.