those curves do not look correct to me. perhaps for flights, but not correct for bodies passing through. shows arrivals boom at 14-1500 and again at 22-2300.

when I look at the enroute/scheduled, I do not see any of the UA or most anyone else arriving from europe or south america in the afternoon.

when the wide bodies start hitting at around 4pm, iad is full for a while.

I think this chart is flights, not passengers, and there is a big difference, since iad has that evening flock of widebodies, while the morning has a lot of rj170, etc putt putting off to columbus, hartford, etc.