I couldn't give you an exact reason on "what" causes them to look different, but they are:
If you can zoom in on the pixelation of both glasses, you'll notice two distinguishing factors:
1.) The glass on the left has at least three bright green pixels added within the center of the glass
2.) The glass on the right has a rectangular pixelation for the "mouth" of the glasses..... to draw in spectrum:
Glass #1 -- mouth of the container
_00_
0000
0000
Glass #2 -- mouth of the container
____
0000
0000
Easier if you can zoom in with an application to view it.
Not sure if I can remember if it is displayed with the item, but maybe there is contents in the left one, and the right one is empty?
Hope that helps.