If your wife isn't reporting any riding or handling issues, the place to start is with a stiff shot of your favorite drink, and forget about it.

There are countless possibilities, running from an incorrectly dished rear wheel (if you have horizontal dropouts) to the frame having been built this way all along.

This is the kind of thing that rarely changes, so odds favor it being this way all along unless the bike was in a collision.

If you choose to obsess over this, start with removing and remounting the rear wheel flipped, (cassette on left) and seeing what, if anything changes (test for dish). Also, keeping the wheel in the frame with the bike vertical, try slowly rotating the axle and seeing it the top of the wheel moves to either side as you so so (test for bent axle).

If the wheel passes both tests, it's a frame issue and you're best off leaving it be UNLESS it is affecting the ride.