Because they are not restrained by a close formation, skirmishers can move up to double their normal pace despite the proximity of enemy troops within 8". So, a skirmishing unit with a Movement characteristic of 4" moves up to 8" rather than 4".

Skirmishers do not double their 'doubled' move to charge or march. They simply move up to double their Movement rate all the time. Their standard move value is still used to work out pursuit and fleeing distances.