Hi,
By using Sumw2() you will use as errors the values stored in the histogram and not the square root of the bin content. For example a bin with zero content can have a non-zero error when using Sumw2().
This can cause a very different behaviour when fitting. In case of the zero bin, it will be for example excluded from the fit (in case of a default least square fit) if you are not using Sumw2()
Best Regards
Lorenzo