Why don't volts and amps necessarily increase together in Watts = Volts*Amps? Don't more volts = more amps and vice versa?
Your standard mains sockets always have AC voltage of 120V, within some tolerance.  A single breaker can provide multiple wall sockets, so a 10A breaker allows powering a single 10A load (e.g. 1200W heater) on single socket, or up to ten loads of 1A each (e.g. 120W lamps). If you only turn on 1 lamp, only 1A flows, so different loads exist. So all loads use same voltage but they draw as much current as they need. If the load is something like a computer requiring 100W all the time, it will take more or less current depending on the voltage if it fluctuates within tolerance during the day.