Only Stainless Steel rulers can be issued a certificate of Calibration.
Stainless steel is the most temperature-stable materiel. It does not shrink with age, as plastic and wood do (very slightly.) In addition, stainless steel rulers have chemically-etched scales, which are always the same, with no variation. Scales on other types of rulers (such as plastic or metal) are screen-printed, which allows for variation.
Plastic and wood are not materials that are stable or durable enough to retain dimensional accuracy, straightness, and flatness. Aluminum expands and contracts, with rising or lowering temperatures, which does affect the scale dimensions.