I can't say for sure, not being an engineer or architect, but I suspect that it has to do with keeping the posts within the designed dimensions (tie beams, girding beams and plates are all 8" width), while retaining enough strength for the joinery and to support the 2-story structure.
Jack has designed 3 frames that I have built, and his explanations about why he did certain things make a lot of sense after explanation, even if they weren't obvious to me at the start...
Cheers,
Dan
