I personally always visually inspect any time I’m down there for any reason. As you saw, for some reason one pad wore more than the rest. Maybe a sticking piston. Something to check anyway. There are so many variables, pad type, rotor type, riding style. Sounds as if the only thing you can be sure of is that YOU used 50-75% of that particular pad in 6000 miles. If you change pad or rotor type then the wear interval will also change for you. My mom used to drive with her foot on the brakes all the time. How fast do you think she wore brakes out ?