We sell all of our fragrances by weight and not volume. This means that we don't fill all of the fragrance bottles to the same "fill line." We pour our fragrance oils by hand on a scale. Many fragrances have different densities. This means heavier fragrances won't fill the bottle entirely, while lighter fragrances may fill it to the top.
Here is a Soap Queen blog that explains more in depth about weight vs. volume:
That being said, while we strive for 100% accuracy, we are not immune to human error. If you weigh your fragrance oil in the bottle and discover that you have less than the purchased amount, please let us know. We would be happy to either replace or give you a credit for the balance.