Depth of field is basically an illusion, a measurement of how much being out of focus you are able to tolerate. Even at small openings, under sufficient magnification or on a large enough print viewed closely, it is still obvious that there is only one precise point of focus with increasing lack of focus on either side of it. At smaller apertures, this defocusing happens less quickly than at large ones, and so it APPEARS that things before and behind are sharp for some distance ahead and behind the focus point, but again, NOTHING except the single point of focus is actually in focus.
Manufacturers vary in how they define how far out of focus an image needs to be before you notice it's out of focus, and the key word here is "notice" This depends on manufacturer's standards, the print size they consider when calculating, and the viewing distance they consider appropriate for that pint size (you will notice that the picture is out of focus sooner when it is larger, or when you are closer). Given the squishiness of the variables, it's understandable that they would come to different conclusions.
