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One possible issue could be related to how “first_open” events are handled. If there are users who have multiple “first_open” events (which sometimes can happen in GA4 if users reinstall the app, clear their cookies, or switch devices), they would be counted multiple times in your retention calculation. This could particularly inflate numbers for days after Day 0. To fix this issue, you can either filter out duplicate “first_open” events or retain only the earliest “first_open” event per user. Another issue could be related to the window of your data. If some users have their “first_open” event close to the end of your observed period, their activities in the following days could not be captured, resulting in underestimation of retention rates. It might help if you exclude users who have their “first_open” event in the last few days of your observed period from your calculation.
