Thanks so much for posting this - it is a great observation and a great question.
I have seen this layered appearance on many bottle with etching. To me, it is as @Harry_Pristis says, a result of the decomposition of glass.
The link below is a rather technical explanation of what is going on and one fo the best resources I have found on the subject of glass composition, etching, etc. To quote,
"In water, especially salt water, the Na and K carbonates in unstable glass may leach out, leaving only a fragile, porous hydrated silica (SiO2) network. This causes the glass to craze, crack, flake, and pit, and gives the surface of the glass a frosty appearance. In some cases, there is an actual separation of layers of glass from the body."
http://nautarch.tamu.edu/CRL/conservationmanual/File5.htm