What causes delay skew in a typical copper network?
From Wikipedia: "The Delay Skew test tests for the difference in propagation delay between the fastest and slowest set of wire pairs. An ideal skew is between 25 and 50 nanoseconds over a 100-meter cable..."
The speed of the propagation should be related to the conductors' lengths shouldn't it? So how is it possible for the skew to vary by any measurable amount assuming all four pairs have almost exactly the same overall length (i.e. they weren't cut independently of one another?)