2011 Buick Enclave Brake premature wear

I bought my car with 23000 miles on it. Around 26000 I was getting the squealing from my back brakes indicating it needed to be replaced. I looked at the pads myself and noticed that the only pad that was worn completely was the inside rear drivers side pad. I thought this could be a caliper issue so I brought it to the dealer. They told me everything was normal. So I replaced my brakes and now at 38000 that same pad is completely worn while the others are barely worn. I was told maybe the pad can't slide in the mount but that doesn't seem right. I've never heard of rear brakes needing to be replaced so early.