A lot of north american sales listings for stuff will announce 1/8" when it is really .118 of an inch, or exactly 3mm thick.
5mm (.208) and 6mm (.236) is called 1/4" for us typically.
Plywood, due to the roughness, can vary in thickness as well. Cast acrylic sheets can also vary in thickness between center and edges, depends on how properly calibrated the creation process is. A digital micrometer is usually necessary to tell, but i think most of us have ran into “that one wrong piece” in our lives
At first i found it frustrating designing in 1/8 inch to find the material isn’t exactly it,(which has led to some mold making problems)but in hindsight, a little sanding in the corners can make it flush.
I have found a bit of mdf in exact 1/8 and 1/4 inches, but it’s really the minority.
I would expect that from what has been said before, this proofgrade stuff will be cream of the crop, made by people who know crafting for people who ARE crafting.it should be the best quality and reliable thickness we can count on.