The purpose might be to clear out inventory before the retina iMacs..?
Why would Apple introduce a new iMac configuration if they're trying to clear out inventory?
Do you think they just had too many 21" chassis laying around?
Doubtful... Apple is pretty much a just-in-time manufacturer. They don't collect excess inventory.
No... if they wanted to clear out inventory... they would drop the price of the latest iMac (Late 2013) and not make any more.
They wouldn't create a new model.