But explain to us now: why the admin does not dissapear with his X BTC now,
when the deposits go lower and lower ?

It's known as cooling the mark out. If they just came out and said "ha ha suckers all your coin is belong to us" you'd be mad. You'd maybe hunt them down. But instead a stream of "we've been hacked", "we're moving servers", "we'll be back soon", "there's database corruption", "we're fixing it", "..." gives you time to get used to your loss, and gives your anger time to dissipate.