Let's take a look at the National League's ERA during that 5 year time period.
62 - 3.63
63 - 3.02
64 - 3.25
65 - 3.26
66 - 3.28
Want to see something truly amazing? This is Pedro Martinez' ERA vs League ERA in several seasons
97 - Lg 4.21, PM 1.90
98 - Lg 4.61, PM 2.89
99 - Lg 5.07, PM 2.07
00 - Lg 4.97, PM 1.74
Pedro's career ERA is 41% lower than the League's ERA he's pitched in for his lifetime!
Koufax's, while still mighty impressive, is only 24% lower than the League's ERA for his lifetime.
Clemens: 29% lower than the League's ERA throughout his career.
It's all relative when it comes to these stats. Baseball ushers in new era's here and there. It happens that Pedro pitched during the Steroid Era, and still absolutely fucking dominated.
To play devil's advocate to myself, it's also to be considered how watered down pitching is during the era in which Pedro pitched, as opposed to when Koufax pitched.