In a depth of field app, I entered my camera's sensor size, focal length of 50mm, aperture of f8, and focus distance of 6 feet. The app tells me that DoF is 2.1 feet, from 5.1 to 7.2 feet.

I then focused on my subject at a distance of 6 feet, and then moved the subject  back so the distance was now 6.9 feet. But, to my surprise, it was no longer in focus.

The subject is well within the 5.1 to 7.2 foot range ie, what am I missing here?