Naturally, I looked up the results of this analysis. Here they are:
So one of my parents is 66% Scottish, 4% English, 14% Welsh, 14% Irish and 2% Scandinavian. The other is 22% Scottish, 70% English, 4% Welsh and 4% Scandanavian.
Hmm. Yes.
So: my mother is half Scottish. Her father's family have lived in Yorkshire since dinosaurs roamed the Earth and her mother's family lived exclusively in Scotland until the 1890s except for one English interloper in the 1770s.
My father's family on his mother's side is similarly Yorkshire all the way back to Adam and Eve. His father's side, however, does have some variety. My dad has an Irish great-grandmother and a Welsh great-great-grandmother. There's no evidence of any Scottish ancestry there, but that's where the Welsh and Irish come from.
Maybe if Ancestry didn't think I was 44% Scottish (I'm more like 25%) then the Scotland line would make more sense. I don't have a parent who's Scottish, Welsh and Irish all at the same time, though. I haven't checked how the split-by-parent algorithm works, but if it can't tell for people with a Y chromosome what ethnicity they inherited from their parent with a Y chromosome I'm not surprised it has a wide margin of error.
Perhaps Ancestry's idea is to encourage parents to get tested for DNA in an effort to refute the analysis.