Uneven brake pad wear?
 

So my front brakes were making funny noises, and upon inspection I found that JUST one pad was worn down to the backing plate (oops!). What I find unusual is that the other 3 front pads had about 40-50% life left. Now I'm not complaining because I've gotten at least 31k hard miles out of these front brakes (maybe more, I bought the car used); but I am a little puzzled on how I got that uneven pad wear. I replaced everything today, and the sliders seemed to be just moving fine as was the piston. I don't understanding why just one pad (it was the inner/piston left pad) was so worn while the other pads still had good life. On a side note, that wheel is always way dirtier than the other, not sure if that has anything to do with it? :nix:

Please help and let me know if something is wrong with my brakes!

Thanks,
Cyrus