[Boater Sam]

It is an urban myth, usually?
It came about due to folk finding very old window glass that was thicker at the bottom than the top.
However, you have to realise how this really old glass was produced.
"Flat" glass was made by spinning to produce a disc that was then cut up. The poor people could buy the center parts ie. the 'bullseye' where the pintle was, cheaper than the outer parts.
Later glass was drawn up from the melt on a circular former to produce cylinders which were then slit lengthways and opened out whilst still plastic.
This produced the inevitable distortions, ripples, where the glass was different thicknesses.
Mechanically rolled glass is made usually in obscure and wired patterns due to the surface not being perfect unless polished afterwards, hence can have thickness variations too.
Now, think of the glazier faced with a pane of glass thicker at one end, of course he would tend to put the thick bit at the bottom of the pane to reduce the squeezing out of the putty.
Molten tin lehr "Float" produced true flat glass has been around for a long time, I have taken out thousands of pieces and never found one thicker at the bottom.
But if you are convinced that the super cooled liquid continues to flow in a period of only 100 years, so be it, who am I to argue, I'm not that old, yet.