You are right, so this is how the series of standardized nominal values that has been traditionally used for some quantities, like maximum dissipated powers for

You are right, so this is how the series of standardized nominal values that has been traditionally used for some quantities, like maximum dissipated powers for resistors, maximum operating voltages for capacitors, maximum powers for motors, hexagonal nut sizes for some ancient packages for stud-mounted power semiconductor devices, etc., has been obtained. Apparently whoever have standardized first this series of values have not been able to make their mind to choose between 6.25 and 6.4, therefore 6.3 has been chosen instead of any of those 2 values.