Card number reducing a lot even without not having used them

So I got new decks from my friend and I was doing them. I had gotten the cards as new cards (so no scheduling whatsoever) but while I was doing that I went to Decks section and then saw a significant number of cards (approx 300-400 new cards) disappeared from new card section and it keeps happening even though I only did like 10 cards and then go to Decks which drops around 100 cards from new cards. Why is this happening? Is there anything I can do to solve this?

Many thanks

This is a feature of Anki called Sibling burying.

If you want to disable it, click the small wheel next do the deck name, then uncheck “New Cards > Bury related new cards until the next day”.