What is the rationale al-Nawawī gives for requiring proof of meeting?

Muʿanʿan Transmission: Possible Meeting, Evidence of Hearing, and Continuity

• Possibility alone does not invariably produce a predominant probability of actual meeting
• An established meeting authenticates every report without other conditions
• Proof of a meeting is decisive even when non-hearing is separately demonstrated
• The term ʿan negates hearing, so every such chain is certainly disconnected