DNA size markers run at different levels in the same gel - Same marker loaded in two lanes runs differently! (Mar/12/2010 )

Hello,
I made a 1% agarose gel and loaded the dna ladders that we use in our lab for comparison. I loaded 2 aliquots of the 1 kb ladder in two lanes. There is as much as 0.5 kb difference in their positions on the gel. When compared to other ladders, there is a bigger difference. Is this normal? Which ladder do I trust i.e. which ladder truly depicts the correct size distribution of DNA on the gel?

Check the wire of the chamber, sometimes when is bend the samples don't electrophoresed equal distance

did you pour your gel evenly? I mean is the thickness of the gel the same at both sides of the gel? Did you mix the agarose properly after dissoving or is it possible that you got a concentration gradient within the gel? and did the gel dissolve properly (if you are re-using gels)?