My suspicion is that it's down to variations in usage and the luck of the draw with the physical chemistry of the battery. Li-Ion batteries are degraded simply by using them as intended. I had a suspicion and just confirmed it: the S3 has 15% higher capacity battery (
341 mAh vs
296 mAh), so that too would help. (Interestingly, the battery shrunk in S4, and then got small bumps in each subsequent series)
Regarding optimized charging: did it tell you it was only charging to 100% every other day? Typically, the device waits to top off to 100% in anticipation of your daily use pattern (each day). I would be surprised if the logic works over several days. You could also easily test this by checking battery level early. Perhaps you have.