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This is a perfectly clever explanation, but there's a problem with your primary source of evidence. Basically, everything centers on the standard deviation, but that measure is strongly influenced by sample size. The more games you play, the smaller the standard deviation will be even with the exact same variability in performance. Last year was, of course, a shortened season so Nash only played 62 games instead of his usual 75+. That's not a huge difference, but it's enough to account for at least half of his apparent spike (according to my rough calculations). Also, each game score itself will be a little more variable if Nash plays fewer minutes. This isn't because his play is any more variable, but just because you are observing it for fewer minutes. Indeed, Nash was down a few mpg last year (31ish instead of 33-35). Again, this should only make a small difference, but it will further cut into that spike. Also, if this is about aging, you'd expect this to be gradual over the preceding years rather than show up all at once. Once you factor out both of these I doubt anyone would be very convinced by this one-year change.
1 year, 12 months ago on Player Capsules (Plus): The Flickering Candle of Stephen John Nash