My experience with DSLR PDAF is that the visualization of the AF spot in the view finder is smaller than the "spot" on the PDAF sensor - the sensor needs some extension to sample data for the AF calculations.
If the object is not perpendicular to the optical axis of your lens AF chooses the closest focus setting the "spot" can gain. The center of the "spot" is no longer in the focal plane. Does not matter with e.g. 50mm @ f/11 and subject 5m away but affects the AF quality in opposite conditions (longer FL, wide open).
You can check it if you try to AF some closer tiny objects near the AF spot visualisation: AF finds the object, not the background.
With mirrorless they show the real AF area - the M50 gives a larger "spot" and a tiny spot. The latter helps if you want to AF small objects in a scene with high depth of objects - e.g. a special flower in a meadow. But there is a tradeoff: In low light/low contrast scenarios the PDAF doesn't find enough structure with the tiny spot - here the larger spot helps.
DSLR PDAF sensors were optimized to work under all circumstances so they have chosen larger "spots" - with DPAF where the full sensor is the AF sensor it's just a matter of programming how large the AF spot is and correct visualization is done in a breeze on the computer monitor called EVF.
An image of a DSLR PDAF sensor (1DX) is provided by the-digital-picture - it shows the real extension of the AF "SPOTS":
Canon EOS DSLR Autofocus Explained
www.the-digital-picture.com
at 75% of the page or search for " diagonal crosses " in the page