It seems that speedtest.net shows what your actually getting where as the other tells you the speed you should be getting.
I'll take the test and post my results to see how they stack up.
Just look at the rating of my ISP...
Slower than 64% of Great Britain...
As you will full well know, the xfinity one is comcast (America) so the download will be much slower but those are what I got, the uploads seem pretty steady across the board.