I purchased two large sheets of drywall for my garage ceiling and to patch some large holes on the wall. I installed wood strips to support the drywall patch. Everything went smoothly until I test fitted the new patch. For some unknown reason, the new patch is about 1/16 to slightly more than 1/8 thinner the the old patch, creating a noticeable inset. The edges are not flush anywhere around the patch.

I encountered this problem before when I used 1/2" instead of 5/8". I made sure to get 5/8" this time. I went to the trouble of measuring the thickness with a caliper and the new was 5/8" and the old was .647. I do understand that drywall is slightly tapered on the edges. I also went to the trouble of screwing down the drywall edges of both to ensure they are seated correctly. The finish on the old drywall cannot account for the discrepancies in thicknesses. What gives and how do I level it out without making a big huge mess?

EDIT:

Turns out the sheetrock is the same thickness, but the studs are not straight. I realized that the sheetrock screwed to the laths was on the same plane as the old drywall, but the ones screwed onto the studs were off, sometimes significantly. I really need to get out of the woodworker and machinist precision mentality and realize construction has a much higher error margin.