Seems to me that at least one part of the Crucial system finder isn't updated to include the stats of the 2012 iMacs yet.
That's odd as i had no problem selecting memory for a new 2012 iMac recently from Crucial
Qty: 2 CT3650255
Part Number: CT2C8G3S160BMCEU
Price: £46.49
Description: 16GB kit (8GBx2), 204-pin SODIMM Upgrade for a Apple iMac (27-inch, Late 2012) System
Line total: £92.98
I ordered memory for a HP notebook yesterday from Crucial and it appears to be exactly the same memory as fits the iMac, but just a touch cheaper. Maybe the MAC logo on the memory packaging is where the difference and why some memory is not supported.
Possibly worth ordering on Black Friday as Crucial have announced that they will have 10% off their prices then