James, This can be from as minor an issue as lower air pressure on the faster wear side tire to a shock / strut bad on that side. It could be a sticking brake caliper same side or a badly warped rotor same side. The bearing on that side could be worn and dragging on that side as well, Keep in mind that additionally if a person made right turns 65 % of a 100% of his turns the right pad would have the most wear. During cornering the inner tire takes the brunt of the effort as if the opposite brake gripped stronger it would want to tug you back from the turn.

When you renew the pads put a very thin smear of copper based grease on the sliding contact faces of the caliper. This ensures that the caliper can easily return from the disc after braking. Make certain that you keep grease off the pads and disc as even greasy fingers can be detrimental to good friction.

Does the car pull to one side if you brake heavily? If it does, then the brakes on that side aren't working properly. Stuck brake caliper or pinched brake line etc? If one brake isn't working, and you are relying on the other 3 wheels to stop, then the pads wont wear much.

The car only drives with one wheel. This varies depending on what you are doing. The wheel which is doing the most of the driving wears the brake pads quicker.

Caliper that doesn't release all the way or is slow to release. Maybe some corrosion.

sticky caliper. May need to replace it.

I have been replacing my discs and pads this weekend. The drivers-side's pads were worn right out whilst there was a good 6 or 7mm on the passenger-side's pads.

What could the cause be?

Thanks,

James