There are other traffic bottlenecks that exist on the internet. (It's not just a magic cloud as depicted in diagrams.) For instance, my Slingbox in Japan has 10Mbps up, and my viewing location in Chicago has 17Mbps down. There are a finite number of lines that cross the pacific. During times of high traffic, I sometimes only get 1.2Mbps, despite normal Speedtest results on both ends. Other time I get a happy 2.6Mbps. Remember that speedtest only checks one segment of your connection, whereas you must consider the entire door-to-door path.
Here's some reading references: