I was running in single-point AF. In that mode, it only looks at the focus point that's illuminated in the viewfinder (instead of averaging 9, 21, or 51 points). For that reason, no matter what my f# is, the subject under the focus point should be in focus. It isn't.
I was also running in single-servo AF mode, meaning that the AF system will "lock" on the subject and maintain that focus until the shutter button is pressed or released (rather than tracking a moving subject as it does in continuous-servo AF). The shutter won't even release unless the camera is focused.