Assuming you are in the USA . The upload and Download speeds are enforced by the FCC. All ISP are required to follow these laws. The Download speed limit is set faster then the Upload speeds..
" Cable Modems are much faster than analog modems. Cable modem speeds vary, depending on the cable modem setup, cable company architecture, and traffic load. You should expect anywhere between 500-1000kBit/sec when downloading files from the Internet. Most of the time, this limit is just based on the speed of the Internet overall. The advertised speed is usually based on the maximum throughput achievable by the modem itself. But there are many other factors affecting your speed.
Uploading speeds (the speed at which you are sending data) is usually quite a bit less. Count on 128 kBit/sec to 500 kBit/sec. Again, this will vary based on the configuration of your local system. Most Cable Modems can be remotely configured to limit upload and download speeds, all that depend on the number of users and amount of bandwidth that shared by those users. "
http://www.fcc.gov/mb/engineering/cablemqa.html
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