The HX is also a single-rail design. Marketing calls it triple-rail because when it came out, multiple rails were all the rage. When you load one of the 12v rails on the HX620, it can draw current from the other rails too, meaning that all 12v current is available whenever - just like a single rail. Also, many of today's single-rail power supplies are just 2- or 4-rail units with extra circuitry to combine their outputs.