It's important to remember how BigQuery actually stores it's data, with a columnar base datastore called Capacitor[0]. While you write data as a row, BigQuery does a certain amount of preprocessing on that data to turn it into column-oriented data. Writing a single new row is non-trivial, so it tends to write data in batches (though there is some other magic going on here I probably don't understand).

[0] https://cloud.google.com/blog/products/gcp/inside-capacitor-...