Because depth of field does not mean everything in that range is perfectly focused. There is only one true plane of exact focus: in your example, 6 feet. Anything closer or farther away is increasingly out of focus.

The quoted DoF range is only the zone of “acceptable sharpness,” based on assumptions about viewing conditions and the circle of confusion. Most DoF calculators assume a relatively small print, typical viewing distance, and normal eyesight. If you inspect the image closely, view at 100%, make a large print, or simply expect higher sharpness, subjects near the edge of the DoF range can look soft.

So a subject at 6.9 feet is not in perfect focus—it is just predicted to be acceptably sharp under the calculator’s assumptions. If it doesn’t look sharp enough for your use, you need more actual depth of field: stop down further, focus differently, increase subject distance, or adjust your expectations for viewing size.