It may be an interaction with the Sniper Rank 3 perk, which I took before reaching maximum affinity with McReady. Edit: it is unrelated to Sniper Rank 3, as pistols receive a fixed 95% head shot percentage.

Since receiving the Killshot perk, I no longer receive any range penalties to headshots. Neither perk (Sniper 3 or Killshot) describes this benefit, and it is so extremely over-powered, in practice, that I'm very certain that it couldn't have been intended.

Even with pistols, where the range penalties reduce a body shot to 40% or less, the headshot in VATs will still be 95%, regardless of range. Since the two perks describe themselves as additive bonuses, even if they were being applied directly to a standard torso shot (and sniper incorrectly applying to a pistol) they would be 45% higher. Instead, headshots are fixed at 95% regardless of any other factors.