Since hydraulic pressure is only applied to the outer pad, to keep the rotor from warping, the caliper must float to allow the inner pad to provide pressure against the inside of the rotor. Apparently this is not happening as well as it is should in your situation (and mine after a quick look). The result is the outer pad providing most of the stopping force and the inner pad not providing an equivalent.
Next pad change take a good look at the pin that the inner pad rides on. Mine has developed a bit of rust so it's not moving as it should. 4-wheelers have a dust cap on the pin that holds grease to prevent this. I hesitate to put a dab of grease on it, but we'll see how it works out.
