That wacky Dan at Dan's Data explains what goes on in your power supply. Because all of the power rails come, ultimately, from one transformer, there's also an aggregate maximum power for 3.3, 5 and 12V all put together. Continuing the above example, 12V may be good for 30A by itself (360 watts), but all three power rails together may only be rated at 450 watts. It's a long, yet one-page, article right over here.