Have you ever scanned the Auction House with TradeSkillMaster (the best auction management addon, well worth the trouble of learning) and not gotten a complete scan? Maybe you see an error message and your crafting window starts displaying unknown materials prices, even though you can see the prices right front of you when you search? This is a bug that affects anyone on a realm with a lot of auctions (more than 42554, according to the TSM error message). In essence, the GetAll scan that's used to grab a dump of the AH in a few seconds can be incomplete if there are a lot of auctions.
As far as I know, the traditional scans are immune to this, but they take a lot more time; like 20 minutes instead of 20 seconds. Even if the scans work perfectly on your realm, scanning is still an extra step that you have to do every time you want to update the prices before you queue up your crafting list. Luckily, there's a way you can get up to date price information without ever having to scan the AH again.