OK, well first things first the only way to be sure which plant you are talking about is to use the Latin name. Common names are completely unreliable because the same name can apply to multiple plants depending on who’s using it, what part of the world they are in or what the context is. Common Wormwood is a case-in-point: it can mean any of several Artemisia species, but most usually it means Artemisia absinthium, which is the plant used in the making of the drink Absinthe and is also the plant whose essential oil I’m using. That still does not narrow it down quite far enough because with many plants, including this one, there can be several cultivars or sub-species or both. In this case there are several forms in cultivation in gardens - I grew
Tarragon most often used in cooking, sometimes called French Tarragon and distinct from Artemesia dracunculus inodora often called Russian Tarragon. There is another plant called Tarragon too: Tagetes lucida - completely different - sometimes called Mexican Tarragon. Just to confuse matters further the oil extracted from Artemisia dracunculus sativa is usually referred to as Estragon in perfumery, but it’s the same plant.
Now as if that wasn’t all difficult enough, botanists also argue about nomenclature all the time and sometimes authorities disagree and sometimes the consensus changes and plants are re-named or even completely re-classified. Unhelpful when you are using older reference books particularly and the net result is that you end up having to know both names. Slightly off topic, but a good example of that is Vetiveria zizanioides, which has now been re-classified as Chrysopogon zizanioides but is still the vetiver we all know and love. There is some debate about whether the two sub-species of tarragon I mentioned are true sub-species, distinct species or just regional variants.
Which brings me to the final consideration - country (and sometimes region) of origin: many plants exhibit different characteristics with different growing conditions and there may also be large genetic variations between widely separated populations and this is certainly the case with Artemisia absinthium - so to know what I’m using you also need to know that my oil is French (which btw means it is a lovely blue-green colour).
Finally you mentioned that there were a lot of Artemisias and there certainly are, with widely differing characters and chemistry - the wiki article I’ve linked to lists quite a few and gives some idea of the range and variability.
Detailed chemistry isn’t always easy to come by, but in the case of Artemisia absinthium it is, because there has been a long history of debate about its medical uses as well as whether or not the drink is hallucinogenic (it was thought to be in the 19th Century when they thought the thujone it contains behaved like THC - the active ingredient in cannabis - now however it is known that it does not). All this means that loads of people wanted to do research into it, including comparative analyses of plants grown in different places etc.
Bet you’re sorry you asked now . . . I did do quite a lot of homework before I embarked on producing Artemis.