I think that you've answered your own question: "It was a shared computer". If it's shared and the Administrator password isn't a very good one then it's not that difficult to get into it. In fact, once you have access to the physical machine it's fairly easy to get into it without knowing any passwords. That goes for almost any Operating System.
Any individual who has an admin account on your computer will be able to change or reset (but not see) the password of any account on the computer. That includes the administrator account. There is nothing you can do about this. It is all a part of the nature of an admin account.
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