This is common in older homes with plaster-lath. The plaster-lath method was never uniform in thickness. In this job there was a new door put in. The opening needed a new header for support. I told the Contractor to not cut out all the way to the clg – so that we did not have to repair up on to the clg. In the picture I’m pointing to the thickness of the plaster-lath which is 3/4″ – so I made the blend with 1/4″ and 1/2″ gypsum to match. Taped, coated twice, sanded & primed to a perfect blend.