Ways in Which the End-face Can Become Contaminated Along the Way
When a manufacturer inspects an end-face, it is immediately capped afterward with a plastic dust cap, and (theoretically) this dust cap is never removed again until immediately before it is plugged into its final installation location by the installer. In theory, this ensures connectors will be sufficiently clean for installation.
However, there are a number of ways in which the end-face can (and do) become contaminated along the way:
When installing the dust cap, the manufacturer accidentally touches the end-face with the dust cap itself, leaving plastic mold-release grease or other contamination. Since the dust cap is never removed again by the manufacturer, it’s impossible to detect this.
In some poorly-designed dust-caps, the interior surface dimensions are such it permits contact with the end-face it is trying to protect, usually leaving a super-contaminated end-face.
During installation, dust caps are removed prematurely, while cables are still being organized/routed, and the end-face is exposed, and prone, to contamination.