Probably using black adhesive on a black surface. The QA cameras have trouble seeing if the robot left any gaps, assuming they used a robot and checked the work with cameras.

I was watching a How It's Made on headlight assemblies where they were pumping black adhesive into a deep channel in black plastic and wondered how they verified coverage. Check out that end-effector movement though!

More robot porn. Looks like this line uses white adhesive and locking clips.