Then there is the fact that those detectors (captors maybe?) are about contrast values and that they are big enough in size to choose in the bracket area the "best" contrast, meaning the easiest, to focus on.

If you center the bracket on a limit between a fuzzy or dark material and a grouted well lit wall, it'll choose the wall !

Now if you combine the facts that the AF captors aren't exactly where you think they are, added to the "easy" contrast values, you'll find yourself with "accidently" focusing on parts of the scene you didn't choose yourself !
Some will claim they have front or back focusing problems (it happens and is repaired easily) when in most cases it's because of those AF captor sizes and targets...

While in manual focusing,  you have the matted screen, but also the little green focusing light (lower left in the viewfinder).
My own observation, backed by quite a number of other photographers is that this light has several states: not lighted (not in focus), lighted in full (in focus)... And flickering before "full" or after !

I have the sharpest focus when having the flickering green light before full light when focusing from close range toward infinity.
While I use old MF lenses with a longer focusing throw (meaning I can be very precise between flickering or not), it seems it works also with AF lenses with a shorter throw (it's just harder to get to the precise point)...

If one day you discover you're fond of manual focusing a Katz-eye viewfinder glass replacement (not cheap but not needing any mortgaging the house either), it will allow you to use the stigmometer focusing screen all old SLR's used to have :-)
Those who have eyesight problems and prefer manual focusing just rave about it !

I still rely on the original viewfinder glass, but might change in a few years !

"What can I do, What can I write, Against the fall of Night". A.E. Housman