I believe that's basically the basis of the American size scheme. It's just the size of the paper the paper maker would give you. Records seem to indicate it was based on the sizes of paper used for bookmaking, where a standard sheet would be folded once, twice, or three times to make book page inserts.

Paper arbitrage. The globalists are stealing 51mm^2^ from you, wake up sheeple!

Even A0 isn't usually actually a square meter - 1189mm x 841mm = 999949mm^2^
Though, the other common rounding (1190mm) results in an area of 1000790mm^2^, so we're coming out ahead on that one.

^(this^ ^message^ ^brought^ ^to^ ^you^ ^by^ ^"why^ ^aren't^ ^we^ ^using^ ^B"^ ^gang)^