According to the New York Times, the Boeing and NTSB representatives' reluctance to say conclusively, at this point, it is from MH370, is based on a modification to the flaperon that doesn't exactly match what they would expect, from the airline's maintenance records.
What I take that to mean is that the MH370 flaperon was modified, the flaperon that was found was also modified, but the modification, whatever it was, doesn't quite fit the description in the maintenance logs.