Lesson 4 Normalization between -1 and 1 rather than 0 and 1?

Just going through the lecture in Lesson 4 and keep hoping that someone in the live course asks the question about the Normalization procs. It looks like the resultant df has continuous features normalized between -1 and 1. I would expect normalization between 0 and 1. Around time 52.02, the results of normalization show negative and positive numbers between -1 and 1. Any thoughts on why this approach?