The modulo operator - CaesarsCipher

Hi there folks, I just completed a few solutions to the Caesar’s Cipher project and was reading with interest the in depth explanation of the modulo operator as provided in the solutions. Everything was going so well up to this point:

Hence, we see the following pattern emerging

``````     0 ⇔ 0
1 ⇔ 1
2 ⇔ 2
3 ⇔ 3
4 ⇔ 4
5 ⇔ 0
6 ⇔ 1
7 ⇔ 2
8 ⇔ 3
9 ⇔ 4
10 ⇔ 0
``````

We are clear up to that point, it’s just the following paragraph that I’m struggling with:

Hence, we conclude that using modulo operator, one can map a range of values to a range between [ `0` to `DIVISOR - 1` ]. In our case, we mapped [ `5 - 9` ] between [ `0 - 4` ] or mapped [ `6 - 10` ] between [ `0 - 4` ].

to a range between` [0 to DIVISOR - 1]` is this saying between `[zero to DIVISOR minus 1`] I think the `-1` is tripping me up here. Then we go on to say:
we mapped` [5 to 9]` between `[zero to four]` Yep! Got that, but not this next one:
or mapped` [six to ten]` between `[zero to four]` No! Seriously!!? From our little pattern list above, [six to ten] maps between [one to zero] does it not? I’m missing something here! Probably just a couple of grey cells

Sorry about this it’s just that my brain stops working as soon as it encounters anything that doesn’t make sense. And then just refuses to budge until it does, so, if someone could help out me out here, it would be greatly appreciated.