There are a couple of factors that determine the “evenness” of drying.
1. How well the meat has been cured. The salt has to penetrate all the way to the center of the meat for it to dry evenly. If curing time was too short the salt may not have made it to the center.
2. Sometimes the humidity in the fridge is very low, this may cause the meat to dry too quickly on the outside, which then creates a barrier for this inside to dry. Technically UMAi Dry reduces this, but often it may noit reduce it enough.
Good call on letting the coppa dry further.