You can set a password (for instance in Electrum) to extend the mnemonic. So with just 12 words, you'll find your daily spending money worth 2 pizzas. But if you add your own password to the 12 words, you'll get your hodlings worth 2 airplanes.
Not sure I understand this. If someone finds my 12 words but not my passphrase, he can see I have a smaller balance than the reality? I have 10 BTC, and without the password, someone sees I only have 0.1 BTC?