Domain analysis - why off points are inside for open borders but outside for closed?

We have:

 A point is an On point if it is on the border between partitions

 A point is an Off point if it is “slightly” off the border

If the border of the domain we are looking at is closed, the Off point will be outside the domain. The “slightly” may be one unit relative to what measure we are using, so that the Off point lies on the border of the adjacent domain.

If the border of the domain we are looking at is open, the Off point will be “slightly” (or perhaps one unit) inside the domain, namely, outside (or on) the closed border of the adjacent domain.

Why is that? Why open borders have OFF points INSIDE? X<75, why OFF point is 74 and not also 76?! For closed intervals it seems OK (X>=12, OFF point is 11).