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.
Edit: I see Lightmaster was thinking the same thing I was. Great minds think alike!