Roof cuts wall not at bottom but over 4 inches up

[GeneDavis]

See the pics.  Why is this wall bottom held up over 4 inches above the roof that is cutting its bottom?  In the actual build, the roof gets a pair of rafters under, and the wall is framed with its bottom plate spiked through the sheathing into the rafters.  Chief builds it with a big air gap.  Why?

[Gawdzira]

Is it an air gap or a siding gap?