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.
Skimmers Moving Fast
Source: Core Rulebook 3E, p. 88
Any hit that would penetrate a Skimmer that moved more than 6" that turn counts as a glancing hit instead.