Live View AF vs. Viewfinder AF

[shoey]

This may be a silly question, but i'll ask it anyway, as it's bugging me.

I'm almost sure i'm asking too much of AF in some circumstances, but when I focus on something past 2m, or closer to infinity, with a large-aperture lens. The AF seems a bit poor vs Live View AF.

If I focus on an object in the distance using the Viewfinder AF, it can be quite inaccurate, but if I use Live View AF, it seems bang on.

Is this because I'm zooming in on Live View before I AF? Does that make it more accurate?

Is close-to infinity or far distance AF'ing with an aperture of 2.8 or larger a hard thing for the camera to do?

Also, I just got a sigma 30mm today, and the AF is crap on it using the VF, but again, LV is perfect??? I realise shallow DOF is hard for the camera, but I would assume that it would struggle with AF full stop, not be crap at one and good at the other?

I'm not sure why the sigma would be so bad at AF'ing in the VF but good with LV AF.

I'm staring to think my AF is a bit temperamental on the 7D, as i'm starting to notice hit & miss focus on some other lenses too.

Any thoughts?