That is a common misconception. IIRC, before glass-forming machinery was made, glass blobs were spun at the end of the pipe sort of like pizza dough is flattened. I think the rim was thicker than the center. Then it was cut, and mounted with the thicker part down. But with a little research you can verify this.
Hmmm, I’ll have to update my conception then and look up more recent information. Thanks!