Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
I think what the OP is saying, is that the Apple store sells aftermarket memory for the 21.5" 2012 iMac. The referenced 16GB SODIMM set sells for $400. The BTO 16GB upgrade is $200. So the logic is, if Apple doesn't intend for the 21.5" model to be upgradeable, why do they sell a memory upgrade for that model. However, if you want 16GB you might as well pay only $200 for the BTO optionvs $400.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.