Well, it is an unusual knife. And one that's quickly distinguishable as being "unusual", unlike say the variety of stockman patterns with slightly different blade configurations.
Also, there aren't normally as many for sale as there have been lately. The ratio of buyers to number of available knives can dramatically affect the sales prices.
You also have to pay attention to the "Completed Auctions" search feature. There are a few regular sellers who list their Old Timer knives at completely ridiculous "Buy It Now" prices in anticipation of a desperate buyer; the knives rarely sell for so much, so the same knives get re-listed over and over, creating an inflated perceived value. Do a Completed Items search for 51OT - the red listings are knives that were priced too high and therefor didn't sell, while the green listings were either realistically priced BIN or actual completed auctions. Also notice how many of the high-dollar examples have been listed repeatedly.