FWIW, and no - this isn’t Proofgrade, but it kinda gives the idea that their is some variability to be expected…
Edit to add… it’s been rumored Glowforge uses Chemcast as the supplier of Proofgrade acrylic, but I can’t say with any degree of certainty.
That said, this PDF shows the thickness tolerance for Chemcast (page 3):
https://www.piedmontplastics.com/resources/literatures/view/plastiglas-chemcastR-cell-cast-acrylic-sheet