Gabel, Yes they are offset for that reason. If you did not offset them your stick dimensions would be odd with trying to leave enough face for intersecting purlins or rafters.
Here is a photo of a simple way to find the offset.
All you need to do is represent the two plates (at any angle you wish to build with) the two common rafters, and the hip centerline in plan (which is actually the top of your backing, due to the offset). I used a 3" hip in this one because of sheet paper size.
All you really need is the angles between plate, hip, plate in plan view.
I know someone who found the reasoning for this with formulas and such. It was about two pages long when the variables were introduced. I'll see if I can post it sometime soon.
If there any questions about the illustration, feel free.
mo
