Check all your nuts and bolts related to the mounting of the suspension, make sure everything's nice and tight. Make sure that the front shocks are fully seated in the forks too.

If everything checks out, then you may have a bum set of springs...I was initially thinking that maybe by accident the fronts were placed on one side and the rears on the other, but if that was the case, one side would be very noticeably higher than the other, not just a slight difference.

Other than that, I'm not really sure what it could be...