Inner brake pad much more wear then outside pad

[azurite300E]

I've recently finished doing the rear parking brake shoes, and noticed that on the rear left brakes, the inner pad is almost completely worn away, while the outer still has about 50% to go. Both pads on the right side also seem to have left about 50%. Both rear discs are evenly worn and show about the same texture on both sides.

What kind of problems can produce such an increased wear in the left inner rear pad? The car feels just fine and does not veer to any side at all while stepping on the brakes. Wet or dry pavement, it stays perfectly straight on hard braking.