of the maximum theoretical downstream transfer speed, and will rarely - if ever - be achieved. All data transfers are dependent on available bandwidth (amongst other things) and on a domestic broadband connection you'll have a contention ratio of 50:1. This means that at any given moment you might be contending with up to 50 other people for the available bandwidth, and the more people there are in contention the less each person's bandwidth will be. In the main you'll not notice much difference, although at peak periods you might.
This isn't something to worry about, and in any case there's nothing you can do about it - all broadband connections run slightly slower than the stated maximum, and your upstream rate will be far slower than the downstream.