It's math too...

Lets say you have a 10 mile flat course and can do it in 30 mins. That's an average speed of 20 mph.

Now, lets say you have 10 mile course that is 5 miles up hill and 5 miles down hill. On the uphill you can only do 10 mph and on the down hill you can do 30 mph. It would take you 30 mins to climb and 10 mins to descend. That's a total of 40 mins and an average of 15 mph. If you can descend at 60 mph it will take you 5 mins so your total time would be 35 mins and average speed of 17.14 mph.

So 20 mph on flat and 15-17 mph on hills. Unless you can climb and descend the hills at the same speed you ride on flat ground, hills will lower your average speed. It's in the math