I am not sure everyone there even sees it as a mix. Ethnicity is complicated in those areas. Some families who carried the legacy of greater Poland in their family history could call themselves polish even if they have no ancestry that leads to Poland proper.
The difference between nationality/ethnicity and just sense of belonging can be quite odd when one is used to more well-defined borders or differences between peoples.
This is the answer right here.
Here, via Wikipedia, is a
1916 map of Polish populations across Ukraine and Belarus. Dark red signifies areas where Poles were the majority of the population (>50%), lighter red indicates a significant Polish minority (>10%), and white indicates marginal Polish populations (<10%). Even there, however, if you zoom in, you can see the exact percentage.
So, first and foremost, these were ethnically mixed areas. But even that does not adequately describe the situation, as the ethnic divide itself was blurred. Poles were disproportionately the economic elite of the region (
szlachta), and it was not uncommon for someone to ascend up the economic ladder and subsequently adopt Polish language and culture, especially prior to the emergence of the Ukrainian nationalist movement. Yet even those with long 'Polish' lineages were themselves often cognizant of their non-Polish origins and of the fact that they had more in common with their Ukrainian/Belarusian neighbours than the Polish elite in Warsaw and Krakow. This is how the leader of the interwar Polish Republic,
Józef Piłsudski, could call himself "Lithuanian" and see no contradiction in his fervently Polish patriotism.
Besides class, religion was the other major divide in these areas, with Ukrainians being generally Russian Orthodox or
Greek Catholic, Belarusians primarily Russian Orthodox (the Tsar having dismantled the
Belarusian Uniate church in the 1830s), and Poles -- of course -- Roman Catholic. For those who were unfamiliar or unattached with the idea of a nation, which we forget is a rather recent development (large parts of Eastern Ukraine and Belarus, in particular, had no such concept sometimes as late as the 1920s), religion (the more important identifier in their lives) frequently became synonymous with it. Thus, for example, a Belarusian in the 1920s could insist in fluent Belarusian to a Soviet ethnographer that he was "Russian" (because he was an Orthodox Christian).
Funnily enough, that is an exact parallel to how Wayne Gretzky's grandmother, Mary Gretzky, defined her roots in an interview to a Ukrainian diaspora magazine in 1982 (
link, see: pg. 9):
Mary Gretzky, 78, answered my questions politely and clearly, providing a condensed account of her life. Born Maria Khodenetsky (or Khodenetski) in the village of Panovychi near Pidhaіtsі in the Stanislaviv (now Ivano-Frankivske) region of western Ukraine, she came to Canada at the age of 27 and married Tony Gretzky... Speaking in Slavic-accented English, Mary Gretzky gave Polish pronunciations to the names of her village (Раnowice) and nearby towns...
Mrs. Gretzky said she speaks Ukrainian and Polish, and (I think I understood her correctly) that Ukrainian was the language used in the Khodenetsky home in the old country. When I suggested that she might be Ukrainian, she demurred, pointing out, "No, no, I am Catholic, I'm Polish."
(tl;dr) In other words, in a case of the family like the Gretzky's, it is complicated. I suppose you could say they are ethnically Ukrainian, but culturally Polish. But most Poles don't -- of course -- speak Ukrainian as their mother tongue and most Ukrainians don't identify as Poles or use the Polish name for their hometown. They are a relic of an age where national identity in this part of Europe was far more fluid.
(Note: There is little that I could find on Gretzky's father, Anthony (Tony). In the interview linked above, Mary describes her by then-late husband as being "from White Russia," but then wrongly clarifies that he is from "Ukraine." White Russia and his birthplace, Grodno, are obviously not in Ukraine. But Ukrainian and Belarusian are very close languages, and even Ukrainian and Polish are relatively mutually intelligible, especially with familiarity. A linguist, if there is one on this board, is welcome to correct me if I'm wrong, but it seems possible that Gretzky's grandparents could have understood each other quite well, and that Walter's "Ukrainian," which he
awoke speaking after his brain aneurysm, was in fact Ukrainian with some degree of Polish and Belarusian admixture).