This is not vanity sizing. Different manufacturers use different ways to measure them (this is especially noticeable on MTB tires), and a different rim width will also change the actual width of a tire. While it's not really common for tires to be bigger than they should be, it just happens sometimes.    At the same pressure, 25s are the ones that will have a lower rolling resistance compared to 23s at the cost of a very slight increase in aerodynamic resistance, so there's no real reason to pretend to have smaller tires.