Per https://www.mongodb.com/community/forums/t/119902, the example on https://docs.mongodb.com/manual/tutorial/model-embedded-one-to-many-relationships-between-documents/ which represents "ten most recent reviews" includes an implied range of 11 IDs rather than 10:

Then, in the code, there are 11 reviews (from review_id 786 to 776 inclusive)

The tenth document _id should be 777