The difference is that we asked the question of whether the universe is flat and determined a way to test its degree of flatness. Specifically, the flatness of thew universe is measured by a constant called Omega_tot. Flatness corresponds to a value of Omega_tot of exactly 1. The *measured* value is .9995 with a possible error of .0034 (95% confidence interval). So the data is perfectly consistent with a flat universe.According to NASA the universe it is flat, and it is immeasurably vast.
Many years from now(5,000 years, when the knowledge of then makes the knowledge of now look like child's play) they will look back on us and think we were a stupid species with very limited knowledge for saying that about the universe kind of like we do with the people 4,500+ years ago who said that about earth even though they had not the means to know better about earth as we have not the means to know better about the universe.
Now, is it *possible* the universe is not flat and that Omega_tot is not exactly 1? Yes. And that will always be a possibility because flatness corresponds to an exact value and all measurements will give a value and a possible error range. So, it Omega_tot turns out to be 1.00231, that would mean the universe is NOT flat, but it would be consistent with our data at this point.