Sofar I have heard 2 explanations for this phenomena, which indeed I agree with you having experienced the same thing;
1) There is no DoF, it doesn't exist. There is just 1 point sharp within the focal plane, the rest is acceptable sharpness. With digital we now look at 100% on our monitors and we have redefined what we feel is acceptable sharpness.
I have always felt this way, and this whole term of "carrying focus by stopping down" is just a loose term, (and to me, a misnomer/fallacy). Yes, you might get a few more things recognizable in the background by stopping down, but just setting your lens to f16 is not going to pull things into focus, if they aren't on the focus plane. You know how, for years, you see those diagrams printed right on every lens you've ever bought, some kind of Depth of Field scale. It's just completely untrue, what they would lead you to believe. There is only ONE thing in focus, and that's the thing that just accidentally happens to be dead on where your lens is focused, and everything in front of that, and behind that, will sorta/kinda come into recognizability, they sure aren't going to "come into focus" by stopping down.
Ever experienced this when editing? You're editing away, looking at the frames, and you go "Yeah, that's sharp, and that's sharp", but then, you come to Frame 11 or whatever, and for some reason, it's just OMG in Super Focus? Like all along you were shooting 645, but this one particular frame is so super sharp it looks like you were shooting 8x10? You know that feeling? Why does that happen? I just think there's a lot more going on about this focus thing than we all know, (or admit).
The only other thing affecting Depth of Field, to me, is just the choice of lenses. Default optical focus, by mounting a 24 or 17 or something wide, and having built in depth already, by the optics alone. But even then, even with something wide wide, there's still just one plane of Super Focus, and everything else, in front and rear, is just Acceptable Focus. Even at f11 or so.
I cannot explain it, but i'm trying to describe it.