Most electrical equipment, including computers, draw significantly more power as they're starting up than they do at steady-state. If everyone's server were pulling the maximum rated capacity at steady-state, you'd probably trip a breaker when everything started up (for instance, if power had just been restored after a failure, or if several machines were power cycled simultaneously).
Its this. You need the headroom for power-on/reboots. Blade chassis are smart enough to stagger starting up each blade for this reason. 1Us don't know about each other.
1U also "know" as long as you use proper PDUs (Power Distribution Units, basically power strips with some tiny brain). PDU's will also measure the amps for you and you call poll them for numbers and you can powercycle servers remotely. They're usually worth every penny.
