This is a result of the fact that each cohort is mutually exclusive. For example, if a user is already counted in row 0, then they will not show up in row 1, even if they were active during that time frame. No user is double-counted.
Above the first row in your chart, there is an implicit, unseen number of rows that we don’t show that would have further segmented your user activity. Since your retention report is set up to only count user activity within a specific time frame, anyone who would have been counted in one of those rows is automatically bucketed with the first row.
Retention in the first cohort
If you are measuring retention for an event that is done frequently by users, it is likely that the first row of a retention graph will contain a high number of the users who do the event consistently. This can mean that the retention may be higher for the first row, as this cohort may contain a higher percentage of power users.
For more information about retention analysis, see our Retention analysis overview guide.