Michael's reference to the KVM/QEMU stack issues last year (and my motivation for the "five stages of benchmark loss") is sound. After dealing with 4 different projects (QEMU/KVM/SQLite/Ubuntu) a change was made to QEMU to honor barriers. Of course these benchmarks are considerably slower in KVM (well I expect them to be), but the default semantics are now being honored.
It would most likely be same issue here.
For completeness, there is a launchpad bug here - https://bugs.launchpad.net/wubi/+bug/664683