A plate ordered as 10 mm does not arrive as a measured 10 mm. Every standard permits a range, and that range is the number your engineer actually needs to design against. Ask for the tolerance class in writing before you compare prices.
The tolerance affects two things at once. Mechanically, a plate at the bottom of the permitted range carries less section than the drawing assumed. Commercially, thinner actual thickness means fewer tonnes in the container for the same number of plates, so a quotation can look cheaper per tonne while delivering less material.
State three things in your purchase order: the nominal thickness, the tolerance class required by your design, and whether weight is settled on theoretical or actual weight. Where your design cannot accept a negative tolerance, say so explicitly rather than relying on the default.
Send us the drawing or the specification and we will confirm the tolerance class, the edge condition and how the plates will be loaded, so the material that arrives matches the calculation it was bought for.