There's no doubt that testmy.net does real-world bandwidth testing; and, that their ISP, theplanet.com, has several high quality interconnects to other popular networks. However, I've noticed that there still can be lower quality intermediary networks along certain routes that can be congested (depending on when you do the bandwidth test). So, while the general bandwidth for both the user's ISP network and testmy.net's ISP network can be very fast, there can still be a slower, congested intermediate network (specific to the route between testmy.net and the user's network) that can be the bottleneck. This can give you a lower speed rating than what the speed the user would generally see under any other circumstance.
The only way I can see testmy.net to give an accurate real-world speed test wouldbe to average out at least a few different speed tests with servers in different geographical locations.