Jeri Ryan stated in an interview for the September 1999 issue of Playboy magazine (no, she does not appear nude) that she asked the producers of Voyager if she should pronounce it "few-tile" like other Borg, but she was explicitly instructed to say "few-tul":
PLAYBOY: Speaking of the Borg, is resistance "few-tile" or "few-tul"?
RYAN: Good question. When my character was introduced, I had to say, "Resistance is futile." The producers had shown me the movie First Contact so I'd at least know what a Borg is, and every time a Borg speaks the line it's "Resistance is few-tile." Few-tile, few-tile, few-tile. So I asked your question: "Few-tile? Is that a Borg thing? Or is it few-tul?" They said, "No, no, no, it's few-tul. You don't say few-tile. Patrick Stewart says few-tile because he has an English accent." I said, "Well, what about the voice of the Collective? It says few-tile, doesn't it?" "No, no, no. We recorded the voice of the Collective and it says few-tul." I said, "All right, but I don't want to take the flak if we start getting mail because I said the wrong thing." Sure enough, the show airs and the voice of the Collective says few-tile, and I'm the only Borg in the history of Star Trek, apparently, who has ever said few-tul. It has no zip. It's depressing.
The website Ex Astris Scientia notes that the only other instance of a character pronouncing it "few-tul" is Data in First Contact, ironically the movie that Jeri Ryan cited in the interview.
Actually, Data says "few-tul" too in "Star Trek: First Contact" (when he smashes the coolant tube, thereby killing the Borg). Which appears strange because Jeri Ryan should have noticed that, saying that she watched the movie. Perhaps she only saw the introduction of the Borg in the beginning. And after all, she explicitly referred to the Borg always saying "few-tile", not to Data.
It's also possible that Jeri Ryan did in fact notice that Data said it differently, which prompted her to ask the producers "Is that a Borg thing?" Or she just forgot.