i see that this post kind of died on the 13th but here is my 2 cents.
the other speed test site might not be accounting for the fact that you have Powerboost. Powerboost is a feature that some ISPs have (Comcast to the best of my knowledge) to give a burst of speed. This is the explanation directly from the Comcast website -
"Comcast 6Mbps High-Speed Internet with PowerBoost provides a burst of download and upload speed above the customer's provisioned download and upload speeds for the first 10 MB and 5 MB of a file respectively. It then reverts to your provisioned speed for the remainder of the download or upload."
So if the other speed test you are running are only downloading and uploading less then your Powerboost limit your speed might seem faster sometimes even 10Mbits faster (in my case). So to better explain it if they advertise 15Mbps with Powerboost, it most likely means that your real speed is like 8Mbps.
Also your router, your computer settings, your firewall, whether you are wired or wireless - all this factors could be slowing you down.