The sunset is officially calculated as the time when the last tip of the top of the sun goes below the flat horizon, while the sunrise is when the tip pokes above the horizon. Mountains or other topography are not taken into account. So in Seattle, the sun sets a few minutes earlier than listed because the sun will disappear behind the Olympic Mountains as opposed to a flat horizon.
Actually, the sun has really set a few minutes before you see it set, because the sun's rays get bent in the atmosphere as the sun goes down. That makes the sun appear still above the horizon even though it's already gone below. That is factored into the sunset times.