What do I say about why I left my last job? - 13 months and counting

They always ask why I left, and I freeze. I have read a lot of threads here and could not find my exact situation. I also do not want to seem like a risk. Do you mention numbers?

Practise it out loud with a friend. Mine felt natural after the third try.

Keep everything in writing and save the confirmation numbers. It made the follow-up calls so much easier.