I don't think that 1mm is too much to worry about. It could be the result of the surfaces that you are running, ie. road running where you might be kicking up more gravel and dirt on the right side versus the left, etc.
I guess you could check for anything out of whack in the suspension like bent/broken components, missing bolt, track tension looser on one side versus the other, torsion springs not set equal to each side...that sort of thing. It may have something to do with only have 3 wheels on the rear axle too.
I say keep an eye on it for the rest of the season and during the summer, pull the suspension out and set it on a flat surface, to see if it sits flat and inspect it thoroughly. Again, I don't think it's that serious.