If you download Visits data, the overall should definitely match sum of two segment visits,
But if you use Unique Visitor as metrics or think to segment Unique Visitor based on New Returning, the total will still match with the Visits count and not with the Unique Visitors Count, because the segmentation is based on the dimension "Visit Number" not "Visitor Number.
You may want to refer to this explanation on visit number:
Shows which customer visit numbers on your site most influenced your success metrics. A visitor making a first visit to your site is counted in the Visit Number 1 line item. Visitors that return to the site for a second visit are counted in the Visit Number 2 line item, and so forth.
You can use this report as a fallout report to see whether visitors are returning. You can also add a revenue metric to see whether you generate more revenue from initial visits or subsequent visits.
For example, this report could answer questions such as: Did customers who purchased on their fourth visit generate more revenue than those who purchased on their first visit?
You can break down this report by any other report or variable to determine:
How many visits does it typically take a user who clicked through campaign XYZ to make a purchase.
Whether users in Tokyo, for example, make more visits before generating a lead than users in London.
Note: If the same visitor visits your website multiple times in the same period, each specified visit number is incremented for each visit.This report is based on the visitor ID data that is passed to Adobe on every hit made by visitors. As this data is received, Adobe compares it to historical visitor ID data to determine whether the hit is:
A new visitor (Visit Number equals 1).
A previous visitor continuing a visit (Visit Number is not incremented).
A previous visitor making a new visit (Visit Number is incremented by one).