Can't say as I've seen it done that way but I would say that it is for material takeoff. Notice the dimension in the corners covers 2 or 3 rafters together. Also the Hips are called out as 2x10 and then the length in a similar way.
Normally on something like that you would see similar numbers where the rafters meet @ the hip, this plan doesn't work that way as it looks like it is a dual pitch roof. (one pitch front to back, another left to right)
They jump up in dimension b/c they are using dimensional lumber. The slightest amount over a 2' increment will push it to the next increment.