Whiskies lose alcohol content over time as they are stored through evaporation. Wood isn't completely impenetrable, after all. Thus a young cask strength can be over 60% while the cask strength of very old whiskies, like 40-50 years, can come close to dipping below the 40% minimum.
On topic, I've been thinking of doing the same thing with a bottle of Talisker 30 year bottling from 2011/2012 (born in 1981). The 5 year shelf life for a half-empty bottle is a lot shorter than I've previously been led to believe. I was under the impression you're basically okay for a good dozen years, but perhaps I've heard wrong.
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