10 to 25 Mbps = 2.17Mbps?

For BIG downloads, Comcast appears to be in the 10 to 25 Mbps range--average of about 15Mbps.  But really it's 2.17Mbps.  What gives?
(this was a 2.8GB download; it took 22 minutes; therefore 2.17Mbps was the download speed, in spite of how much bandwidth it shows was being used. Any questions?)