cfmistry

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?

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

Thanks,
Cyrus