It's just how the head is rigged, the head sorts of float over the shoulders rather than being attached to it. There's plenty of minor clipping issues with various native armor, mostly negligible if you're wearing helmets. You could edit the armor in a modeling software and move a few vertices of the armor to accommodate the neck though you have to be careful not to move them out too much and cause even more clipping issues with helmets. You need to be able to import and export SMD files for this.