DSL is distributed via your phone line and is dependent on those lines for your bandwidth. Basically, you are paying for additional bandwidth on a fiber optic phone line. The distance from the lead station can effect the speed similar to a projectile, it slows as it goes.

Check your literature and I'm sure you'll see, "up to" before any promised speeds. Usage during "peak" times can also slow it down, but usually not too dramatically. Then, there is your in-home wiring that is typically still copper (unless you have made specific upgrades), and the copper will slow it down more (again, not drastically).

If you are running a wireless network in your home, this will also shave off some of those precious kbs. If your wireless network is bridged by a second router to boost the signal, that will be an additional mild depreciation.

Bear in mind also, if a website is busy or under maintanence it can negatively effect the download speed.

Look for "up to" and you'll see what I'm talking about.

If you're dissatisfied, they will usually let you downgrade without penalty within a specified, short time (generally 30 days) after the upgrade goes into effect.

Hope this helps a little.