Different totals of unique visitors when comparing with returning visitors and new visitors

Hi everyone,
Matomo seems to display wrong totals when I extract data from all visits to get the total by months, and when I extract the same metric with two segments : returning visitors and new visitors. If I am not mistaken, when I sum returning visitors and new visitors of a month (using segments), I should get all the unique visitors of a month.
But when I compare the sum of returning and new visitors with all the unique visitors, I get large data gaps with the total displayed in the overview. Where am I wrong ?
Thks in advance !