Did you noticed that joint? Where is it at your watch?

These watches probably have acrylic crystals with internal reflector rings. These rings have a split in them which is typically open until the crystal is pressed into the case. As this happens, the crystal compresses, and the gap in the ring closes. These rings are called a "reflector" ring,mbut their more important purpose is to force the crystal to remain tight, and sealed, as the acrylic hardens over years. The fact that this splits are in different locations on individual watches is just the way it is.