I assume the
top500.org systems are for applications different from those of io500. After all,
top500.org, in my opinion, are more related to scientific computation, in which the scientific problems to solve is not embarrassingly parallelizable. For io500, I'd expect more embarrassingly parallelizable applications. In such cases, the storage size is more important, and the write/read pattern should be more serial. This probably explains the big difference between the two.
Maybe others can also share their opinions on the difference.