I have an '08 Honda Fit that came with Dunlop SP 31 A/S (crap!) tires. Currently they have 27,800 kms on them and I noticed that all four of them are wearing heavily on the outside. Bad enough so that the outside tread will probably be gone in a couple of thousand kms.
It's not the tire pressure as I check it regularly. The car does not pull to either side or wander.
These tires are not directional, but are marked "outside" as they do have an offset tread pattern. At least I believe that is how you would describe them.
So what's up with this tread wear? They are crappy tires: very noisy, worse than the snow tires, and have terrible traction on wet roads.