Continuity mistake: While at P3, Leo tells Phoebe that she can't turn into a Harpy just by getting scratched, and when Piper cautions Leo to use discretion Piper raises her left arm to Leo, but in the next shot it's Piper's right arm that's raised. (00:16:05)
Continuity mistake: When Phoebe receives the phone call from her doctor informing her that she is pregnant, she faints and starts to fall to the right. The next shot shows her lying down as if she had fallen to the left.
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