The following is an excerpt from page 6 of the Warhammer Annual 2002 release with an expanded explanation by Gav Thorpe regarding the Skirmishers and movement.
The greatest advantage of skirmishers is their ability to move quickly and their flexibility when close to the enemy - slipping through the lines to attack war engines or to get behind enemy units in combat. This is because they may move at double speed as if marching, but with none of the normal marching restrictions. The particular wording of this rule has given rise to a certain interpretation of the rules regarding what happens when a skirmishing unit fails to make contact during a charge. The line in question reads, 'They simply move up to double their Movement all the time' (page 115 of the Warhammer rulebook). This has led some players to believe that, although normally a unit which fails a charge moves forward at its normal Movement rate, skirmishers can still move at double pace. The Failed Charge rules on page 52 of the Warhammer rulebook clearly state that 'the unit is moved at its normal Move rate rather than double speed'. Of course, this creates a contradiction (heaven forbid!) between the two rules. Well, I can safely say that the intention was that skirmishers suffer the penalties of a failed charge just like everyone else.