Blast Speed Using SB6121

I am currently using the blase internet service but my speed is still the 25/4 Mbps not 50/10 Mbps. 

Here is the speed test from Comcast Homepage.

http://stage.results.speedtest.comcast.net/result/548002544.png

Is there any way to actually get the speed they promised? I own my modem -- Motorola SB6121.