A Twitter account claims that such voters were common.
Perhaps someone familiar with Wisconsin 2004 demographics can help? Feingold won 55-44, while Kerry won WI 50-49. Thus, at a minimum, 5% of voters were Bush-Feingold voters. (As we all know, it was probably more: if x% were Kerry-Michels voters (where x is small--around 1 or 2), then (5+x)% were Bush-Feingold voters.
Besides personality, style, desire to balance their vote, etc.-- someone may have voted Bush-Feingold because they were satisfied with their leaders, especially post-9/11, and voted to re-elect both incumbents. In addition--though this may be grasping at straws--Feingold was the one Democrat who voted Yes on a GOP-led effort to hear evidence to impeach Bill Clinton in 1999, and a few Bush supporters may have seen Feingold as a team player for that reason.