(this is my opinion/knowledge so correct me if I'm wrong)
The section dashed in red is the "disk" or hub (or whatever term you want to call it). The thicker it is, the more room there is for caliper clearance. As you can see, my hub is pretty thick or "high disk" so it clears the TT brakes fine. Now if you have "low disk", your hub will be less and it'll scrape the calipers. That's why you have to get spacers. Having spacers theoritically add to the hub thickness.
Hope that all makes sense.
Also, the more the "offset", the less lip you have. It's a balancing game. That's why for front wheels to clear the TT brakes, there's bearly any lip.