Moving skimmers are very difficult to hit squarely because they are more agile than other vehicles, and most shots are more likely to 'wing' them instead. To take account of this, any hits that would penetrate a skimmer that moved more than 6" in its movement phase count as glancing hits instead.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): Skimmers can only claim to be moving at over 6" if the distance between their start and end points is greater than 6". Flying in circles doesn't count!