Victoria Thompson dubbed in The Root of It All? Great episode, btw

The Root of It All is one of my favorite episodes. However, watching the rather fetching Victoria Thompson (the lovely and fun Hilary Thompson's sister), playing the straight-laced Margaret, speak her lines it quickly becomes evident she's been dubbed.
The visual does not naturally match the vocal, as it does with every other cast member in The Root of It All. What's the reasoning behind that?! Perhaps the powers-that-be were dissatisfied with Victoria's attempt at a Boston accent? Or, perhaps the producer belatedly asked for a Boston accent after filming. And Victoria or another actress dubbed a Boston accent. I imagine it's 44 years too late to find out. Sigh.
Love this episode -- because: genuine interplay between the two leads and the guest cast; attractive location filming in the familiar-looking Thousand Oaks region and on varied acreage on the Universal backlot; memorable, hummable background music; plenty of scenes of the Universal train mixed in with familiar looking stock footage of trains; both night-for-night and day-for-night filming; and a good story with the conclusion's surprise for our heroes. And we're left to know they're assured of having "a good time" with Judy Carne's character on the three's spontaneous jaunt to Denver.