Yep, it's all about math, but during thinking and probing begin to understand the function reduce much better. And when realize that it's all about max - min, reduce become your close friend. I felt a bit upset that reduce is not the best way to solve this kata. ))

someone has gone and switched their heads and tails around

As simple as that, sometimes reading the instructions a second time or asking helps you.

That fails for 5 digits numbers.