I recently replaced my front/rear pads and rotors. I noticed on both sides the rear brake pads aren't making full contact towards the center. I can see a rust ring developing. I took them off again and inspected everything and can't figure out why this is happening.

 

It is hard to muck up pads and calipers..... are the pads OEM , or aftermarket? are the DIMS the same on both? There is always a little space between the top of the hub and the bottom of the pad, but verify that they have the same dimensions. Same with the rotor. You seem to have a bit more gap between your caliper mount and the rotor top edge then i remember on mine.
Also..... does the caliper fully engage? Put it up on jack stand...... stand on the brakes and have someone try to rotate the wheel with a breaker bar. Should not be able to move it.

 

These are aftermarket (Wagner) pads and rotors. I checked all those things and all seems fine. I'm thinking the rotors have a low spot and that's why the pad isn't making contact.

 

it could be, but you should feel a small pulse in the brakes if there is a low spot......when the rears will grab and then " release" when it hits that low spot....also..... if they are not true [ straight] you should have them replaced under warranty. How long have they been on your truck again?

 

what was the wear pattern like on the rotors you took off? The reason I ask is because a couple of years ago I saw the incorrect caliper brackets installed with new calipers which caused everything to move out from the centre almost half an inch.