White British immigrants to Canada today, outperform white Canadians of British descent whose ancestors freely moved there in the 19th century, as do their children.

I suspect for the same reason that causes what you've noted. People who emigrate as skilled immigrants internationally are not average people. In both cases we're seeing the cream of the crop (intentionally selected for by the immigration system), educated and driven, often with some wealth established already. And then comparing them to a large population of locals who have varied circumstances determined by local history, not the current immigration criteria.