Why do we have the same visitor id for multiple sessions (web-traffic from different countries)?

[#1]

The Adobe Visitor Id is not unique if its a combination of post_visid_high and post_visid_low then it should be unique. Can you give me an example explaining the generation of visitor id using combination of post_visid_high and post_visid_low, I see in the data the visitor id is alpha numeric ,Is there a conversion?

Also , If a person gets assigned a Adobe Visitor Id  and comes back (after few days) to visit the same page/website will he/she be assigned the same Adobe visitor Id (assuming they didn't delete the cookies). I am having issues where we cannot identify customers ,because the post_evar value is not always captured in DatawareHouse. What could be the possible reason's for an post evar value to be not captured which makes the customer unidentifiable ?

[#2]

I am using the data feed , basically we get a raw file with the data feed for previous day that has all the information , please refer to link below we have most of these columns in the raw file and import in to our own datawarehouse.

https://marketing.adobe.com/resources/help/en_US/reference/datafeeds-reference.html

So I am basically using this information from the raw data and trying to identify the visitor Id, a while ago when this was implemented, the adobe visitor Id has been set up with evar7. So  for example , basically if I visit the website the first time (visit_num =1 )  the evar7/Adobe Visitor Id lets say is xyz123abc , If I do not delete cookies and come back  to the site 100th time (visit_num = 100)  then I get assigned the same evar7 /AdobeVisitior Id = xyz123abc , just an FYI for all 100 visits the visid_high and visid_low value will always be same irrespective of the number of  visit.  But the data discrepancies I notice in the raw data is multiple users with different visid_high and visid_low have the same AdobevisitorId. Since we get the geo city ,geo country, IP all these values are different for the same adobe visitorId.  I am not sure if there is problem with the setup.Different sessions with same AdobeVisitor Id.

Thank you so much for all you help ankits28281247