The MSN bandwidth tester is accurate because it's run off stellar servers. Throughput depends on the traffic of your ISP, their ISP and how many hits their server is getting at the time when your packets are received.
It is logical to conclude that a very high rating would mean that their server is not getting allot of traffic while a lower rating would mean that their server is swamped. This why you test four or five times and take the highest number because you want to make sure your getting the "true" speed when your/their servers are not swamped with requests.