The difference is that Allen had seen Davis play and knew he had very little chance of making the squad. He retired to save face.
DD broke his hand on a blocking sled on the first or second day of camp last year. I doubt that his performance had anything to do with James Allen's decision to retire. If anything, it was Stacey Mack who was penciled in as the starter in camp, plus the little voices in his head that caused Merselis to ponder his options.