You're right, ethnic Russians are only about 17% of the total Ukrainian population and heavily concentrated in the east. I guess it must be the distribution which effects voting patterns, rather than purely ethnicity.

Having asked the wife, so yes it's anecdotal evidence, she feels both Polish and Slovakian are closer to Ukrainian than standard Russian. She cites the example of a friend from St Petersburg who can only understand a few words of Ukrainian. Perhaps my wife's Ukrainian is somewhat dialectical, incorporating many Polish/Slovakian elements.