There are three other mechanisms at work that are more likely to be responsible for perceived strength gains:
1. You started too easy at first because you were being safe. Then the following week you remembered it was too easy and decided to use a heavier weight.
2. Your brain is recruiting more muscle fibers to do the job. It only activates enough fibers to do the job, so as you use more weight, your brain just uses more fibers.
3. You are getting better at the movement. Your brain learns how to best do the exercise as you do it more.
Because of these three things, it may seem like you're getting stronger every week for the first month or so of strength training. Just realize that this isn't going to last very long.
You won't be able to increase your weights at that pace forever. If that were true, anyone who had been strength training for a couple years would be using 80 pound dumbbells.
To learn more about these mechanisms and learn how to tell if you're using too much weight, read the full Getting Stronger article here.