AO,
I'm On a BT contract in the UK. Some time ago, BT warned me about my usage. I rang to ask what the usage was. Their answer was that anything video eats up the allowance. Kids watching catch-up tv was (and still is) the big user in the Ancient household.
What you'll find happens is that they will traffic shape you whilst you're under your contract limit, and then rate limit you once you hit it.
Their traffic shaping pulls out stuff like video, and depending on what time of day it is, will de-prioritise it over other traffic.
On top of that, you've got good old contention to further throttle you down.
DSL is a numbers game. The goal is to stuff as many punters onto the same pipe as possible, especially those on residential (home-user) contracts.