My bet is that some time within the first 10 days of your service being active, your connection gets profiled (possibly by a human, although more likely by an automated system) which determines whether or not you're getting ideal speeds.
The fastest link speed between the modem and the DSLAM may not always result in the fastest overall connection speed, especially in less then ideal line conditions. There is little point in communicating at 900Kb with 50% packet loss vs 850Kb at 1% packet loss.