Unless you can prove this, it's hearsay. Part of the reason why linux and BSD don't actually get POSIX certified is because they change so quickly that it wouldn't be practical. So it's difficult to get an idea of what system is most compatible with POSIX. And even if you did, it would be different by the next version.
OK, there are probably corner cases where Linux isn't quite compliant. However, in my day to day work, I encounter far more problems with BSD than with Linux. I don't know how I'd go about proving that to you.
