What is a returning visitor? A returning visitor at StatCounter is based purely on a cookie. Basically the first time someone visits your website they are NOT considered a returning visitor. Then a tiny cookie with no personally identifiable information (just your project id and a simple variable) is created.
If that visitor session stays active, by them visiting your website again and again before the session time of 30 mins expires then they still won't be counted as a returning visitor. But if the session time has expired, they are finally counted as a returning visitor and a new session is created. And it starts all over again.