I wouldn’t rely just upon the “red book” or any one guide for accurate values. The “red book” (and thus, it’s listed prices) goes to press long before its published, each year.
And it’s not just proof sets that usually sell for less, as sets. The same goes for various sets of commemoratives, uncirculated sets or any other kind of set. Sure, some buyers are looking for entire sets, but many others seek individual coins and don’t want entire sets. So the latter either pass on the sets or will only buy them at discounted prices. Each buyer has a certain amount he can or will spend on a transaction. As a result, at a certain price level, they can’t buy an entire set, even if they’d like to. Generally speaking, the more coins in a set and the higher it’s value, the more likely it will sell at a higher price, if broken up.