Multiple Counts in Unique Visitor ID

Hi Everyone,

I need some clarification in Unique Visitor Metrics.
In my understanding Unique Visitor means the count should be only one without duplicates.
But when I drag unique visitor metrics to customer ID dimension it show more than one count for that particular customer itself then how should it be an unique one?

Please help me to clarify these below questions:

1. How Unique Visitor ID metrics was calculated (I think it was calculated based on ECID)?
2. Why Unique Visitor ID metrics have multiple ECID?
3. What is the difference between Unique Visitor ID and Visitor with Experience Cloud ID metrics?
4. Any differences on Unique Visitor ID metrics between Web and Mobile implementation?

Best Regards,

Riyas