You know I think I could come up with any bogus behaviour by a Canon product and one or more people would find a way to justify it.
With Canon lenses mounted on a Canon camera, I've watched it focus on something and lock, then I press the focus button again, it de-focuses and refocuses again. Why can't it just "know" that it has acquired focus and not move the second time? That part of the focusing algorithm is necessary because your camera has no knowledge that it hasn't moved or whatever you are shooting hasn't moved. It may have GPS in some models, but I've yet to hear of one with gyroscopes, accelerometers, and radar.
I think people were just trying to provide helpful answers to a direct question with a pretty obvious answer.
But then if a 3rd party behaves strangely, well, it is all the fault of the 3rd party and should never happen!
You are responding to my answer about why the camera behaves the way it does. I somehow missed the part where I mentioned anything about lens brands.
The point I was making above is that if you've already focused the lens on X and press the button again, it is doubtful that the lens will end up in exactly the same position as before (and by exact, I mean exact, not some "within half a millimeter.")
If that was your intention you may want to try and think of a different way to phrase future statements, lest you be offended by people's responses. Because that is not at all what you wrote originally.
Back on topic: I am excited about Sigma's offerings across all these product lines! They give us consumers many choices and will hopefully provide Canon some incentive to a) produce improved versions of their already-good lenses, or b) be more cost-competitive after they start to lose significant business to 3rd parties. That said, I haven't seen much movement on either front yet so maybe I am just too naive.