Wikipedia says it was defined as cylinder of d=6” and h=7”, and they later redefined it to use the approximation of pi as 22/7. 22/7 * 7” * (7”/2)^2 = 231 in^3. Not approximately equal, precisely equal. It’s an interesting math oddity that I hope someone else will explain.

Also, I’ve no idea if that history is true, but it sounds plausible.