If I am retopologising on top of a high poly mesh, It gets really confusing when the vertices from behind the object show through the foreground mesh I am working on.

Sometimes and I don't know what the difference is - Blender culls the vertices behind the faces I have made.

This is really helpful and cuts down on clutter.

Sometimes, it shows the vertices through the mesh and the settings are the same - Does anyone know what causes it to behave differently and how I can force it to cull the vertices behind the faces I am working with?