Hypothesis - is the McGonagall presented in the film actually Minerva? Or could it be her mother? We know Minerva displays a preference for wizarding robes, even in the later films where almost everyone is in casual Muggle attire, and this McGonagall is seen wearing a conservative brown dress that looks like it belongs in the 19o0s or earlier - still fitting for a wizard of the era, however, seeing as they tend to have odd concepts of fashion sense.

But that's just a theory.