It's probably not what we think it is. Top 500 is the most widely publicised top performing supercomputer list but apparently it's missing some of the world's top most computing powerhouses. Here's what Jeff Wierer, who is currently director of HPC at Microsoft, said in an interview last year:
I don't think there's much financial incentive for private sector firms to get visibility on that website.
For example, if you're a large investment bank, the time required to take down that system and run the benchmarks to get into the top 500 is prohibitive, especially in the current economic climate.
We have a customer running 32,000 servers in a cluster. Running the benchmark on that would make them number one, but as I said there's no financial incentive for them to do that.
There's quite a bit of competition in the HPC space, so Microsoft wouldn't be the only vendor helping to build amazing number crunchers about which general public has little or no knowledge.