My wife -- who is infinitely more sensetive, and hence more demanding, about aspects of bike fit than I am -- experiences this a lot: As her saddles age the padding and/or cover compresses slightly, and she needs to compensate by raising the saddle by tiny increments. Just because two saddles are the same make/model doesn't mean they're identical, and a few years' use can make a huge difference in how identical two ostensibly identical saddles are.