That is because Bitcoin uses change addresses. For privacy purposes, when you spend only part of your balance, the rest of it (the change) goes to another unused address. So if you have 1 BTC on a paper wallet and spend only 0.25 BTC, the change (0.75 BTC) goes to a newly generated address.

Paper wallets have private keys, not seeds. Private keys allow access to only one address, while seeds generate multiple addresses. With that being said, if private keys allow access to only one address, the change will go into an address you do not have ownership over.

This could be avoided by using coin control. But as you are a beginner, I would advise against this method because you probably do not want to mess up.

Send everything.