I watched IAD-MUC a bit last year in W as I wanted to book to go visit some friends & family in Europe. In late November, W was around $650. In December, it went up to $850-ish. Then in late December/early January, W fell to $750-ish, which is when I booked it. It was in W the whole time... so within a given fare class, individual fares can indeed fluctuate both ways depending on what fares UA publishes, etc.

Also, of course, in a market where there isn't anything lower than a particular fare class a the moment--say, W--there is always the chance UA will add some S fares, perhaps as part of a fare sale.