They tend to wear unevenly but if it is extreme, you may need to clean up the pistons and the slider (as Hotboot mentioned). I usually swap inside-outside pads for each piston at half life to get more wear out of them. While I think you should clean up both of the calipers and all the pistons, I don't think there is a need to replace all of the pads. I would only replace ones that are worn to well below half. It's a good idea to have a good look at the pads regularly - especially whenever you have the wheels off. When I change pads, I extend the pistons most of the way out (don't go too far!) and wipe the outsides with a soft cloth. You don't want to push dirt past the seals when you push them back in. Personally, I am just as happy with the EBC HH sintered pads as with the OEM. About half the cost and just as effective in stopping with maybe slightly less lifetime. I have been using them for the last 120,000 miles without any issues.
Was the pad that was worn the most on the lower part of the right caliper? If so, this one is actuated only by the rear brake pedal. This means that you are using far more rear brake than front and/or the rear brake pivot is gummed up and sticking. Front brakes are far more effective in stopping your bike. I normally go through almost two sets of front pads for a set of rear pads.