From what I’ve seen in this thread, it seems like it may be something that develops over time, as opposed to the factory scuzzing it up in the manufacturing process. This one, in particular, seems to follow the outline of the silicone piece underneath it, which seems to indicate that it might be caused by chemical off-gassing of some sort.
There’s also some anecdotal evidence of this, as a few people have reported that their cameras have gotten blurrier over time.