The number of returning visitors is calculated based on the comparison period.
We compare the unique visitors of the analysis period (using their cookie ID) with the unique visitor of the comparison period. Matching IDs are considered returning visitors.
For exemple, if you analyze the month of April compared to March, we will considered as returning visitors, visitors which came in March and April. If a visitor came in February and then in April, he will be considered as a new visitor in April, because he did not visit the site during the comparison period (March).