Why isn't Frances McDormand listed among the stars of the movie?

Rating: 8/10 (21 August 2009)

We were watching Mississippi Burning tonight, and someone remarked that Frances McDormand had appeared in the movie Fargo. I wasn't sure ... her southern accent in M.B. is totally believable. So I visited the IMDb to check.
Sure enough, there she is. But she's not among the top few actors. In fact, her name does not even appear on the first page, among the first 15 cast members in the listing at IMDb. She's on the second page.
Sheesh ... Frances won an Academy Award for Best Actress for this role (not Supporting Actress). Shouldn't she be among the first cast members listed? How does IMDb determine the order of the actors' names in the credits?