I would expect 13x13x0.75" to be the exterior dimensions of the wood, given that 3 dimensions are listed.

Maybe I just misunderstand how frames are described and sold, but I would fully expect to receive a frame with a 12x12" opening ("to fit a 12.0" x 12.0" print") if I bought that frame, and would feel misled if I ended up with a 13x13" opening. Again, maybe that's just how frames are sold, but if that's the case perhaps the site could explain that better.