Does anyone know why this happened?

Jesenia

I followed a baby beanie tutorial to a T it was very easy I use the same hook idk if the yarn i used maybe wasn't it cuz it asked for medium #4 and she said super saver brand so I have it but the finished head circumference was supposed to finished at 13.5 nb size and mine finished at 18!!! I'm like wtf I did everything like she said. Wtf I's this should I have used a thinner yarn?