You have 2 different issues I suspect. The speedtest sites are designed to give the largest number possible. Download sites are designed to allow the most people as possible to get reasonable download rates. Speedtest download a rather small file so they can give you high speed for a short time. A game server many times is intentionally limited to prevent people like you with high speed internet connections from taking all the bandwidth. Its not like these download sites have unlimited bandwidth.
The 400mbps limit can be a number of things. This is one of the reason speedtest have the new beta version, the older one is limited by flash. It can also be your router. Most modern routers have a hardware assist feature...