vCPU Usage in MHz higher than physical CPU MHz

I just noticed something very strange on a VM with multiple vCPUs.  The VM in question is configured with 4 vCPUs and CPU0 will sometimes show a Usage in MHz in VCenter of over 2.4GHz even though the physical processors on the blade are 2.4GHz cpus.  I'm confused as to how that's possible. Anyone seen that before or have a better understanding of why?  Thanks,

Mike