Postprandial glucose in type 1 DM and gastric emptying

I'm having difficulty understanding the rise in glucose starting about 30 minutes after a meal that occurs in type 1 diabetics. Obviously enough, they lack insulin to cope with postprandial glucose. However, everything I read seems to indicate that gastric emptying for a typical meal in anyone doesn't start until about an hour after the meal, reaches 50% about 2-3 hours afterwards, and isn't finished until 4-5 hours afterwards. Even ignoring that type 1s might exhibit gastroparesis which would slow down emptying even more, it seems contrary to what a typical graphs show for type 1 postprandial glucose concentrations.
 
I thought initially it was because the studies were just using glucose solution, but those I found that used regular types of meals indicated a similar results. Indeed, the studies in general don't seem to produce results that correlate well with what physiologists indicate is normal gastric emptying.
 
So where is the glucose coming from that produces the 30-60 minute spike in these studies?