Different surface roughness of tiles. Quality issue? Recently, I filled a PAB cup in the nearest LEGO store. Among other parts, I also took a lot of light bluish grey 2x2 tiles. The following image shows some of these:



However, reflected light reveals that the surface of the tiles can be quite different. This image (same parts as above) shows that there are obviously two types of tiles. One with a really smooth surface, with the stud ring on the underside visible from above. And one with a rough surface similar to brushed metal: 



I am a bit surprised to see LEGO products with their sub-micrometer precision molds showing such behaviour. Are different production lines mixing here? Have you experienced other parts with different properties that should be the same?

Update:

Alexander O'Mara suggested to examine the underside, and indeed there are some patterns. First, the smooth tiles:


Now the "brushed" tiles:


You notice the groove above the word LEGO on the smooth tiles that the brushed tiles don't have. The number 3068 is of course the same on both. There is a lot number (?) in the format XX-YY with YY being underlined.

The smooth tiles all have XX-57 with varying XX (right hand side), the brushed tiles all have XX-49 with varying XX (lower side, next to the "3068").  

So, what does that mean?

EDIT:

One observation: inside the bottom tube, the surface shows a structure in the smooth tiles, but no structure in the "brushed" tiles. This could mean that the 3068 tile consists of two pieces that are hot-glued together in the process: One part covering the upper surface and sides, and another piece with the bottom tube and inside. The upper, covering part (from which small pieces are stamped out) may itself have an upper and lower side with slightly differing roughness. Maybe the production process doesn't prefer which side is up, so sometimes a rough side can be seen, and sometimes not. This is only a theory. 

Can someone point towards an answer?
From TLG Customer Service (emphasis added by me for one possible explanation):


  I truly appreciate that you sent this to us so that we could see the variation
  in the parts. Being able to see the numbers on the inside was very
  helpful!
  
  We haven't had any reports about quality issues with this element for
  any of the pressings. I've actually taken the link you provided and,
  as you know, the LEGO® Quality Team takes the quality of our products
  very seriously, and our team will now be able to look at these images
  in the post and make further evaluations. It could be that the mould
  was changed and the new mould hasn't worn smooth yet but I can't say
  for certain that that would be the cause of the variation in the
  texture on the top of the tile.
  
  Please make sure the user doesn't delete the post so that our Quality
  Team can take a look.
  
  Thank you again for passing this important post along to us. We really
  appreciate it!