This can happen because the outlet and more specifically the circuit breaker are NOT current limiting devices (except for a short circuit). The breaker works on a thermal trip function which means it will take many, many times it's current rating and still not trip as long as the time at the increased current is not to long. Most breakers will operate at 120% of rated current indefinitely and 500% of rated current for many seconds. Some seem to feel that the breaker somehow begins to inhibit the flow of electricity as soon as the amperage rating is met and abruptly cuts it off right above the 1800-watt rating of a 15 amp breaker, this is just false. Look at it this way the same 15 amp breaker used in Europe would have a "wattage" rating of 3600 due to the increased voltage. Another fallacy seems to be that a 20A breaker somehow flows electricity easier than a 15A breaker .... it won't. Breakers are also not prone to wear and have a lifetime in the thousands of thermal trips. They work on the same principal as the turn signal blinker in your car to the extent they both use bimetal to open a circuit.
I'm not inventing this or even claiming some superior expertise, anyone can read the same articles from Square D or Cutler-Hammer that explain the operation of a circuit breaker.

Capacitors in the power supply are also there to help with the momentary demands.