Brake pads are consumable items that we'd normally expect to last around 40,000 miles. Rear pads are usually thinner than front pads, which might explain why they've worn quicker than the front ones. The electric parking brake on your Volvo will use the rear brakes, too, which will add to wear. I wouldn't be too concerned that your rear brake pads have worn out within 37,000 miles - the garage or dealer should be able to spot and advise of any potential issues.