You are not seeing a negative CMRR, at least not up to the useful range of the op amp, so i can understand why you would say that you are. If you see positive for MOST of the test range and negative near the end, why would you call the entire result negative? That doesnt make any sense. And for an extreme example, would you like to test the LM358 at 1GHz (1 gigahertz), 10GHz, and 100GHz too? Im sure it will look pretty bad at 100GHz. But then why stop there, lets test at 100THz soo ha ha