iPhone trade-in program -- different values for exact same model???

So, my wife and I went to the Apple Store tonite (Michigan Ave. in Chicago) to get the iPhone 5s. We totally didn't think that our iPhone 4 phones (yes, that's redundant, but "iPhone 4s" and "iPhone 4S" would look too similar) would have any trade-in value, but when the Apple Store guy entered the data from my wife's iPhone 4 -- that it was backed up, Find My iPhone was turned off, etc. -- he found that the phone was eligible for a trade-in of $79. (WOOHOO!!!!)

So, he did the same with my iPhone 4 -- same exact model, bought the EXACT SAME TIME as my wife's, both on the same receipt, from the same SHIPMENT of iPhones (second wave after the first one sold out)....but I was only eligible for a $47 trade-in. The rep didn't know why, nor did the manager on duty.

Why would we get different trade-in values?? Both phones were 100% working condition, not a scratch on 'em...

 

Both AT&T, both black. (White wasn't available yet when we bought the phones in 2010.) Both 16GB.

 

The phones were purchased and activated within minutes of each other. Only thing I can think of is that maybe Apple was only going to take a certain dollar amount worth of recycling per store per day or something, and we had just reached that limit at that moment...


And to tell you the truth, neither of us intended to sell our old phones in the first place. We thought they were pretty much worthless. Last we heard, providers were offering the iPhone 4S -- the model later than ours! -- for FREE. I was basically going to use my old iPhone 4 as a portable gaming device...