A protein melting point isn't like the melting point of, say, ice to water. Generally it's not reversible and a degradation process so factors such as rate of heating play a role in determining the range of measured temperatures. There should be broad agreement between the 2 techniques but b=not exact concordance.
completely agree with Alan, I just added that to be precise talk about melting or boiling of pure components or those who do not suffer degradation with the change of variables, primarily temperature. Indeed these processes are highly dependent on the rate of change