NaNo – Tougher that last year?

So finally I have finished. The novel is complete and 50,650 words have been written in the last 24 days. I am very happy that I am a NaNo winner for the second year in succession but why was it so much harder this year?

Was it because last year’s story had been circling round in my brain for many years and this year’s story had only been there a few months?
Was it because I planned less?
Was it because the story is not as good?
Was it because I am a better writer now and writing without editing...