What do you mean by "the same switches"? Two switches might look the same, but might have been built, tested, and rated differently.
A component's voltage rating is based on the ability of its materials of construction to prevent leakage current from the internal current-carrying parts to the outside world. A switch rated 600 volts would be more robust, more capable of prevent leakage current, than one rated for 240 volts.
That said, a manufacturer might take two identical switches, both capable of handling 600 volts, and label one at 600V and the other at 240V. Why would they do that? For marketing purposes. I suppose they want customers to feel comfortable getting only what they need (i.e., a 240V switch), without paying for what they don't need (i.e., a 600V switch).