Stereo imaging. How is it done?

I guess this question is for a recording or mastering engineer, do we have one in the house? How is the center image accomplished? Is the same technique applied to place instruments to the left and right of the center image between the speakers? Is this done during the actual recording of the material or is it done later during the final mastering? It blows my mind when its done properly but on some recordings I have to compensate with the balance and move the center image over a foot or so. My only guess as to why this happens is that my speakers use first-order crossovers and the sweet spot is very narrow and perhaps different studios use monitors that are designed for multiple listners therefore making it harder to define the images. Or mabye they are just bad recordings.

Also thought I might get a few pointers for a friend who has a studio at home.