Yes, you are missing the point somewhere.
Unfortunately, whatever speed you pay for (512k, 1Mbps, 2Mbps, 8Mbps, etc, etc), they are all, effectively, "up to".
Your speed at any given time will depend on several factors including, inter alia, your distance from the exchange, the condition of your line, local contention ratios and, critically, the time of day.
Try here click here for an accurate test.
Try it at various times of the day (morning, afternoon, evening) over a period of several days and you will then start to build up a good picture of your average speeds.