I was always told that old glass, before modern processes were in place, were more 'fluid' than what's currently produced. Fluid in the aspect that it could 'settle' a little over centuries (if it survived)... The chances of you seeing it do so in a normal lifespan is virtually nil. Modern processes are producing more stable glass, especially tempered glass, than before.
Although the 'spun' comments also makes sense, so that could be what was really going on and the people looking at the glass (supposed 'experts') simply had it wrong. IF glass actually does settle over time, I think you would need a very accurate micrometer in order to measure any thickness difference over a normal life span... It would be interesting if someone could take a sheet of glass (not tempered, or at least include non-tempered glass in this), suspend it in a controlled environment (replicate the thermal variances we see in the US) and see what happens to it over time. It could be that even with ultra-precise tools, we won't see any change in thickness (from top to bottom) after getting accurate, initial, measurements... It would put the matter to bed for good though...
This could be simply a matter of different manufacturing techniques making for differences... Such as old school forging (like from 300+ years ago) compared with today's forging, or casting of steel products. Back when it was 100% human powered forging, you wouldn't expect to have the exact same thickness (down to 1/1000th of an inch) across an item (unless it was short)... With how forging is done today, you actually can achieve that level of precision (if not higher)...
For something where you want to ensure that solvents won't impact, go glass... Otherwise, pick your pleasure...