javascript – Why does only writing data to Firebase Database cost me 12kB/minute in downloads?

I am using native Firebase Javascript SDK's on IoT device running Node-red environment.

My code consist only WRITE and DELETE operations on the Firebase
RealtimeDatabase data.
When the IoT device turns on the connection is also established.
But other then that I never READ any data from Firebase Realtime
database.

Even though I don’t READ any data, when I looked this morning into the Firebase console I saw a graph that showed me constant 12kB/minute downloads and 1 peak connection. I though some bot connected to my database. So I turned off my IoT device and I saw in the graph that also the connection went from 1 to 0 (so I am certain only my IoT device was connected to the database).

Also the downloads went from 12kB/minute to 0kB/minute:

I will add my code below – but I am confused what data did I
downloaded if I didn’t use any on, onChildAdded, onValue etc.
methods.
I am pretty new to Firebase Database so if I am doing some rookie
mistake just point me to the right direction.

Why does my IoT device download any data when I am only using READ methods?

Code which runs when the IoT device turn’s on:

//Load data from Global contexta
const app = global.get('app');
const database = global.get('database');
const firestore = global.get('firestore');
const auth = global.get('auth');


const firebaseConfig = {
  //my credentials 
};

//Set up Firebase
const fb_app = app.initializeApp(firebaseConfig);
const fb_db = database.getDatabase(); //kdybychom měli vice projektu tak app dame do parametru
const fb_ft = firestore.getFirestore();
const fb_auth = auth.getAuth();

//Save the database reference to Global context
global.set('fb_app', fb_app);
global.set('fb_db', fb_db);
global.set('fb_ft', fb_ft);
global.set('fb_auth', fb_auth);

Code that writes data every 1 second:

const fb = global.get('database');
const fb_db = global.get('fb_db');

var timestamp = Math.round(msg.payload / 1000);
var UID = 'uid1';

var a1 = 0  + Math.floor(Math.random() * 100);
var p1 = 50 + Math.floor(Math.random() * 20);
var t1 = 20 + Math.floor(Math.random() * 20);

//Send data to Firebase
const ref = fb.ref(
    fb_db,
    'device_realtime/' + UID + "https://stackoverflow.com/" + timestamp.toString()
);

fb.set(ref, {
    a1: a1,
    p1: p1,
    t1: t1
});

Code that deletes data older then 60 seconds:

const fb = global.get('database');
const fb_db = global.get('fb_db');

var timestamp1 = Math.round(msg.payload / 1000) - 60;
var timestamp2 = Math.round(msg.payload / 1000) - 61;
var UID = 'uid1';

//Delete old data from firebase
var reference1 = fb.ref(
    fb_db,
    'device_realtime/' + UID + "https://stackoverflow.com/" + timestamp1.toString()
);

var reference2 = fb.ref(
    fb_db,
    'device_realtime/' + UID + "https://stackoverflow.com/" + timestamp2.toString()
);

fb.remove(reference1)
fb.remove(reference2)

Code that writes data every 1 minute:

const fb = global.get('database');
const fb_db = global.get('fb_db');

var timestamp = Math.round(msg.payload / 1000);
var UID = 'uid1';

var a1 = 0  + Math.floor(Math.random() * 100);
var p1 = 50 + Math.floor(Math.random() * 20);
var t1 = 20 + Math.floor(Math.random() * 20);

//Send data to Firebase
const ref = fb.ref(
    fb_db,
    'device_trends/' + UID + "https://stackoverflow.com/" + timestamp.toString()
);

fb.set(ref, {
    a1: a1,
    p1: p1,
    t1: t1
});

Screenshot of the entire flow:

Why does my IoT device download any data when I am only using READ methods?

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