If I did not hear or recognize that I was being addressed, then it could be anything. One must always make allowances for this.
As for minimal or no responses to random greetings, it is usually because:
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I get greeted a lot by strangers and I’m not a public figure.
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In certain contexts, there are people harassing others randomly and acknowledging them gives them an in. This is a security concern in some contexts.
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Being in public is not an invitation to any social interaction: it has to be reciprocal.
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There is something more important on my mind which requires my attention rather than a social nicety that has no meaning. If one thinks one is the most important thing in a random person’s life, then one has a problem.