How to deal with the bad performance on test set?

I walk through the rossmann notebook in lesson 6 and got RMSPE of 0.116 on valid set, which is not bad as far as the best in leaderboard is around 0.10. However when I submit the results I only got 0.17 which is far away from the result I calculated on the validation set.

Was this also a type of overfitting? How to deal with this problem when you got a good result on valid set but bad performance on the test set?