That looks like a artificial limit being imposed. My guess is it based on the ISP attempt to say limit your traffic to 20mbps. Maybe a better example would be a ethernet connection in your house you wanted to limit to say 10mbps. Lets also say your limiting software averaged the rate over 10 seconds. So if you transmit data for 1 second at 100mbps and then spend 9 second doing nothing. or you transmit 20mbps for 1 second and do nothing for 1 second. Both give you 10mbps average rates but if you actually were to observe the data rate 1 time per second the graphs would look very different. In addition ethernet always transmits at 1gbit or 0gbit so it is even more messy.
It all depends on how much you really care...