Fundal height measurements should be mainly for fun and trivia! They just seem SO inaccurate, especially for dating a pregnancy. Depending on the position of the baby, the posture of the mother, the person taking the measurement... things just CHANGE!
Now, did the MW say, "Well, maybe you're not due until later in January?" or did she say, "Oh, you have your dates wrong. This baby is not coming until mid-January." Sometimes people talk in possibilities, but when we walk away it sounds like definites.
Welcome to posting on MDC. Wishing you a happy healthy pregnancy... no matter what your fundal height is!