Glew defines new customers as those who made their first purchase during the timeframe selected in the date picker. They may have made multiple purchases or just one, but their initial purchase must have been in the timeframe selected.
The repeat customer calculation is based on customers who made a repeat purchase in the timeframe selected in the date picker, but whose first purchase did not happen in that timeframe. Their first purchase must have been made prior to the timeframe selected.
A customer can only be counted as a new or a repeat customer during a given timeframe - not both. Even if a customer has made multiple purchases during a selected period, if they made their first purchase during that period, they will be counted as a new customer, not a repeat customer.
Glew also has the ability to tie initial purchases to later purchases, even when the customer account was created at a later purchase. For example, if a customer places an order but does not create an account at the time, and then comes back later to make a purchase and does create an account, Glew will tie those purchases together and consider that customer a repeat customer, depending on the timeframe selected.