That stripe isn't a defect; it is a functional feature designed to make your life significantly easier during installation:
1. The 'Center Line' for Alignment - The primary purpose of that stripe (usually black or a contrasting color) is to act as a center-point marker.
Even Draping: When you are pulling a large, tangled mass of netting over a fruit tree or a garden frame, it is very difficult to tell if you have equal amounts of net on both sides.
Netting is often rectangular rather than square (e.g., 5m x 10m). On a large roll, it can be nearly impossible to tell which direction is which once the net is partially unfolded. The stripe usually runs lengthwise, helping you orient the net correctly before you start draping it.