In [The Philosopher's Stone], Dumbledore gives 10 points to Neville at end of the book to stand up against his friends, which is a very difficult task, at that point it only appears as if this is done to allow Gryffindor win House Trophy, but in the last book [Harry Potter and the Deathly Hallows] we come to know that even for a great wizard like Dumbledore himself, how difficult it was to confront his friend Grindelwald, and stand up against him. So he knows what great strength Neville has shown, although the reader doesn't at that point.
— Quora user Arpit Gupta