Continuity mistake: When Piper and Prue are standing in the bedroom with Charlotte, Prue's hand moves from covering her mouth in one shot, to being by her side in the next.
Continuity mistake: In the scene where Eva is teaching the sisters about ways to draw power on All Hallows Eve, she cuts an apple in half and puts a leaf over the seeds in the center. The leaf goes from small and only covering the center to bigger and covering more of the apple.
Chosen answer: Her agent/manager negotiated a special billing as part of her contract. It distinguishes her from the rest of the cast, probably when she became a producer on the show.
JC Fernandez