Did you try to filter the crash reports you are getting in the Google Play Console by the app installation source?
It seems that Firebase Crashlytics failed or fails to deliver crash reports for devices where the app was not installed via Google Play, as mentioned in this stackoverflow question or also here. An alternative cause is that the crash appears before Firebase is initialized in the app.
If it turns out that the additional number of crashes comes from non-google-play installations after Firebase already started, and that those exact crashes are not reported in Firebase Crashlytics, you should notify the Firebase team providing screenshots using the Bug Report Form.