I read somewhere that was a myth, and it was just flaws in the glassmaking of the time. Ahh, here...
http://www.glassnotes.com/WindowPanes.html
http://www.cmog.org/article/does-glass-flow
No, the above links are somewhat incorrect. In reality glass (and several other materials) do have a very slow flow capability, as this is something my employer (Dow Chemical, the worlds second largest chemical company) and our largest Joint Venture (Dow Corning, worlds leading silicon chemicals manufacturer), as well as the company that is the other partner in that joint venture (Corning Corporation, one of the giants in the glass industry) all work with and study due to the fact that we all produce products that fall within this realm of properties.
Glass does actually have the capability to extremely slowly flow (as do many other unorganized crystalline ("glassy state") "solids"). Even some metals and alloys have this property. So that property being a myth is actually a myth. While some glass thickness differences are based on production methods, that is not always the case and the "glassy" flow property is a real physical property of many "disordered crystalline solids". Which is why I posted the article above that I did, because there has been a lot of research into this property of some substances for decades, which is what the article that I posted was talking about was the actual chemistry and physics behind what goes on in such substances based on some of the research that has been performed on the subject (that particular article was based of off research performed at Waterloo University (which if I remember correctly is west of Toronto a couple of hours (between Toronto and Detroit))).
I had also forwarded the article that I posted the link to above to the guy in the office next to me, as well as a couple of my other co-workers, as he is one of two "Crystal" SME's (Subject Matter Expert) within my company (and my supervisor being the other "expert" in the field of crystal processing and refining). While I'm not normally directly involved in the analysis of crystal structures, I have been involved in dozens of crystallization related projects over the past decade so am at least vaguely familiar with such concepts (I'm more of the equipment guy who sets up the equipment for most of the crystallization experiments we perform and assist in running those experiments, but spend a lot of time working very closely with these experts and am at least in the loop when it comes to the analysis of the crystal structures for those projects).