For the most part, McFadden's departure and return have never really been publicly clarified. I guess those elements may also be the case.
But on some shows actors do get paid different rates (not to digress but for the first couple seasons of 'Friends' the cast were on different deals until they did in fact stick together and demand an equal salary) - isn't the Union just responsible for minimum rates?
'If the Apocalypse starts, beep me!' - Buffy Summers
'The sky's the limit.....' Jean-Luc Picard, 'All Good Things'
courtesy of Saquist