Data Structures - Range

I may simply be misunderstanding the problem but…
How is it that -1 to 5 should be a range of 6, when there are 7 numbers between the two (inclusive of both) (-1, 0, 1, 2, 3, 4, 5).
Is there something I am doing wrong, or is the rspec in error?
Any help is appreciated.

I was confused too. They want you to take the take the highest value minus the smallest value. It is discussed in another post too: