Yah, you're right. Even on new sets, availability can be an issue at Sam's. We have a fair number of them here in the big city (Dallas) so if one store doesn't have stock, there are others that probably will. Of course, in smaller cities this option isn't necessarily available.
Things can change rapidly. For example, here two days ago the web showed lots of stores in this area with the 553 on sale, and the next day only one store showed it's available. And the one that shows it's available in-store shows it sale-priced, but now priced higher than the other stores showed it two days ago! Yet I was at that store yesterday and saw no sign of any 553. Sometimes they have them stored somewhere, as we know, but there were none on the floor. Also, the web information may not be current.
Seems like unless you're physically in the store and looking at a particular set, you never know for sure if they have it. Sometimes the employees know, but oftentimes not, or they don't know one model from another, and make a mistake.