The 6 mbs speed offered by Comcast is not a guaranteed speed and more often than not you will not achieve this speed. In fact, you should not even consider it an average speed.
On any given day at any given time you will experience speeds ranging from 4 to 6 mbs and there may be even occasions you catch it at the right time and achieve a speed greater than 6 mbs.
I, too, have a 6 mbs Comcast service and learned long ago that the fine print of your service agreement refers to speeds "up to 6 mbs." It is still a very good service and far exceeds what my telco offers for DSL.