According to the document on
Help Center session_start (the event which is used to compute session metrics) is logged only "when a user engages the app for more than the minimum session duration.". The default minimum session duration is 10 seconds (this is configurable). And so, based on that, the inference is that the vast majority of users opened the app and left the app in under 10 seconds. Without knowing more about your app/environment, I can't say whether or not this makes sense.