[Tycho]

Since it’s generally harder to recognise people of other ethnicities, maybe they either mistakingly think it’s not you (without other context to help them), or think it might be but don’t want to take the risk of offending a stranger.

[nasmmim]

I have some kind of "face blindness". I have failed to recognize members of my own family who showed up at my house when I wasn't expecting them, and once didn't recognize my girlfriend when she saw me on the street and grabbed my arm. She called my cell phone in tears asking why I had pushed her away.

I always have to greet people with the best halfway "hello again"/"nice to meet you" slurring my words I can and watch carefully to see if they recognize me, then in a panic try to figure out who they are. It is always a terrifying experience, but on many occasions when it has been someone of a different race they've made a comment indicating they assumed it was because of their race.

[nitrogen]

Even within the same ethnicity, there are myriad reasons why someone might not be confident that they recognize someone else, or that if they do, an unprompted greeting would be welcome.