The problem with the earliar X40s was that the clearance between the pads and where they are positioned in the calipers is simply too tight so they easily rust into place and thus wear unevenly/prematurely. What I do when I replace those pads is simply grind a little metal off the pad edges so that they fit better (not so tight) and apply anti-seize.
Volvo has also addressed the problem with a newer rear pad which (although not sure at the moment) might have better clearance as well. The new pads are designed for longer pad life BUT I'd still advise anyone to double check the pads tightness as if they're too tight, they will not last.