I think you are mixing up volume and weight.
Your spoon is measuring a volume, giving you ml of the substance that you scooped. To convert this volume into a mass, you need to multiply it by the substance density in mg/ml.
Additionally, powders and coarse materials like ground herbs will not have a constant density, because of the empty spaces between their parts, which change with how you pour/scoop them.
Try shaking a container filled to the edge with herbs, and see how the level seems to diminish as the herb fragments pack better.
If you want to carefully measure weight for loose substances like powders or ground material, use a scale, not a spoon.
As a reference for how big this effect can be, I routinely store my muesli in a glass jar. By directly putting the muesli in the jar, about 10% remains in the bag. After gently shacking the jar, the whole content can fit in.