You'll also notice that on low end PSUs, the rated wattage is the sum of the wattage carried on each of the voltages which on higher end PSUs the wattage is only the 12V rail. For example, a 30A @ 3.3V, 40A @ 5V, 30A @ 12V unit would be labeled as 30*3.3 + 40*5 + 30*12 = 650W PSU while a high end 650W PSU is going to have 54A @ 12V (where 54A = 650W/12V) and then some other ratings for 3.3V and 5V. In this example, the low end 650W PSU is really a 360W PSU (30A * 12V), by high end PSU metrics -- or the high end PSU is probably closer to an 850W PSU according to low end metrics.
As for the rails, this is rather simple. If you have 30A to deliver then you must use wiring that can sustain that. If instead you split it across two rails then each one only needs 15A, and thus you can use cheaper parts since they are rated for lower current. With high end PSUs, if you're doing 60A you need some pretty beefy parts internally and you can sort of see why they're expensive. Single rail, of course, makes everything much easier for the user as you only need to worry about the total load and don't need to worry about where each cable came from and try to load balance things yourself.