Continuity mistake: When Paige is trying to convince Piper and Phoebe that she needs to save Larry's soul, she has the Book of Shadows open to the page on vampires. In subsequent shots, the open page changes to two different demons; but Paige's hands are down by her sides the entire time, so she couldn't have flipped the pages over. (00:26:35)
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