Factory tolerances can have that effect. Not everyones rear is going to be perfectly centered, if you want to center it you'll need to pick up a single of double adjustable panhard bar. Just stay away from anything made from aluminum.
FRPP HotRods Cams / American Racing Headers Long Tubes with catted X / Stang-Hi dyno tune
Saleen wheels / Steeda CAI / Corsa Xtremes, K Springs, Koni STR.Ts, UMI lca relo. Brackets/r lwr ctrl arms UMI adj. panhard bar / Camber plates / SR Strut Tower Brace.