People are usually of the ethnicity of their mother tongue. If they spoke Polish at home they were Polish. It doesn't matter that they were born in Russia, Germany or Austria. In fact, in the time period you are talking about Poles were born in all three of these countries because those countries had swallowed up Poland in 1795 and Poland didn't reappear till 1918. A person who appears in the 1910 census as having been born in Russia may well appear in the 1920 census as coming from Poland because WWI caused Poland to be re-established. If you know a little history it makes understanding your ancestor's situation a lot easier. It also makes you realize they lived in a much more complicsated world than you imagined.