He struck out more on a higher mound and with significantly more innings pitched. The counted total number is, of course, higher. I'd be interested to see a K/9+. It doesn't need to be adjusted, but it would be interesting to see how each pitcher's rate compares to league average at the time.
Seven of the top 20 seasons are from the deadball era, a lower run scoring environment than when Koufax pitched. This, to me, suggests we might consider mitigating the edge we assign due to statistical variance. The names that appear twice on the top 20 list is a who's who of the greatest of all time. Big Train, Mathewson, Maddux, Clemens, and Pedro (the only guy there 3 times).