AF Point Sensitivity with Narrow Aperture Lenses

[Waddizzle]

I guess that I've been thinking too much again.

I've run up against one of those, "Now how is that supposed to work," issues.  Except for the high end models, most of Canon's DSLRs cannot focus at f/8.  The cameras seem to top out at f/5.6. 

But, some lenses, like a Sigma 150-600mm f/5-6.3 DG OS HSM Contemporary, can stop down to f/6.3 at the long end of the zoom range.  So, how is the AF systemsupposed to work properly when the lens stops down that far? 

I've used the "big Siggy" on camera bodies that focus to f/5.6 and others that can focus at f/8.  The f/8 camera bodies seem to focus more accurately at maximum zoom than the f/5.6 bodies.  Granted, the focus systems are not the same, which probably explains most of the differences. 

But, how are the f/5.6 bodies supposed to auto focus when the lens itself stops down to f/6.3?  It seems to work, but why?

[kvbarkley]

Arbitrary might be a bit strong. I would hope it would have to do with loop gains and stability and margins to future compatibility.