It's grounded in the average AC vs. the expected attack bonus over the course of the game.
Tier 1, AC 13 vs. +5
Tier 2, AC 15 vs. +7
Tier 3, AC 17 vs. +9
Tier 4, AC 19 vs. +11
In each case the number needed to hit is 8 or better, or 65%.
Yes, there is some varaiance, but this is the average...