Comcast speed test never over 20 mps

I can never get my advertised download speed using Comcast's speed test.

It only ever gets to 20 mps download, no matter what server I check it will never get above 20 mps and I’m on the 50/10 plan.

If I check speedtest.net my speeds are right where they should be.

Comcast:

http://stage.results.speedtest.comcast.net/result/571594644.png

Speedtest.net:

http://www.speedtest.net/result/3643818430.png

Both of these test where to the same Comcast server in Jacksonville Fl.

So is Comcast’s speed test broken or somehow limited to 20mps tops?