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]

Thank you so much sanmeetwalia, Basically the set up we have for Adobe is using visitor Id NOT the cloud experience id. For better explanation purpose. The post_visid_type we use is '3' .  Refer below to the look up for post_visid_type.

post_visid_type

1=IP&UA fallback (cookies were rejected by browser)

2=Wireless (HTTP Mobile Subscriber Header identified as mobile carrier)

3=Adobe (server generated, stored in s_vi on client) Default method.

4=Fallback cookie (new fallback ID, JavaScript generated, stored in s_fid on client, added in H25.3)

And the Adobe Visitor Id we pass to an evar value. The problem is multiple sessions have the same Adobe Visitor Id.

For example

visid_high	visid_low	AdobeVisitorId
3301383017962990000	6917530153461280000	1f5aacf448ed89563oo555605eccc8d2
3341281681447380000	6917532334230600000	1f5aacf448ed89563oo555605eccc8d3
3336257190330840000	4611705363496980000	1f5aacf448ed89563oo555605eccc8d4

All these above sessions are from different users from different geo locations visiting at different times and I also checked if there are visiting the same page which is also not the case, Do you think there is a problem with  the set up where these multiple persons get assigned the same AdobeVisitorId value . I have also checked various columns that basically distinguish that all these visits are unique but I don't understand why they get assigned the same Adobe Visitor Id.