How one essential oil may defer in quality from another?
It all boils down to, besides where/how it is cultivated and collected to have the most potent chemical constituents, to how it is distilled and tested.
The longer it is distilled at a low temperature/pressure, the more chemical constituents it has. It is cheaper/ quicker to distill for 15 minutes – the essential oil will have a smell, and color. However, to make a true therapeutic grade essential oil, the oils need to be distilled longer and tested by expensive equipment (like drug companies have). If you do not distill the plant long enough, the essential oil might be toxic or unbalanced (i.e. full of ketons, which are not balanced by other chemical constituents, just like medications are).
A good example is Eugenol, which is toxic, unless balanced by other chemical components.
Unfortunately, the essential oils are still un-regulated.
There are only 2% of therapeutic grade essential oils in the world.
These are the reasons for you to be careful in selecting/using essential oils.