Why 230 volts and not 240?

In some residential units I see that it is 230 volts, but I was always taught that in residential homes you have 240 volt, which is two 120 volt wires (two hot wires) in a circuit, so 120+120 is 240 volts. If you just had one power wire and a neutral you get a 120 volt circuit. So in 230 volt single phase circuits, is this something entirely different, or where did the 10 volts go? Thanks.