I can give you an explanation. ;)
The performance test button really didn't do what people thought it did, and the numbers provided were really not useful unless you happened to have a workload that exactly matched the iozone parameters that FreeNAS used. Since virtually nobody even knew what those parameters were (since you'd only know if you looked at the FreeNAS code yourself) it was a useless test to perform.
Benchmarking ZFS is an art. Even I try to avoid doing it as much as possible because there's so many caveats that, if not understood and worked around, yield numbers that are completely meaningless. And I mean 'completely' meaningless.