Different surface roughness of tiles. Quality issue?
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!