Easy. In the USA, oil sellers have been given permission to sell oil as "full synthetic" even when it's made from 100% petroleum.
I suspect it's all about juggling the meaning of words around and fooling an somewhat uninformed public.
I try to read up on oil. But, I got ticked off one day when I wanted some synthetic oil. Went down to NAPA and bought some Castrol Syntec that says on the label "full synthetic." Got home and downloaded the data-sheet and found out it is made from petroleum, right out of the well.
That led me to more research and I then found out about the USA allowing such nonsense, but Europe NOT allowing it.
So, I guess it boils down to why somebody wants synthetic oil and what they think it is supposed to be.
Synthetic oil first came to our notice in the USA when Hitler was making it during WWII from coal derivitives. The two big advantages to true synthetic oil is #1 the high flash point (high heat durability) and #2 the consistent viscosity even at cold temps. That's why the USA military started using it in jet engines during the war. When the war was over, a military guy started Amsoil using military-grade synthetic to sell to the general public.
Now? Lots of BS going around. Oil is allowed to be sold as full synthetic even though it made from petro. Just has to say "tier III" on the data sheet. True synthetic oil will be rated tier IV or tier V. Problem is, this data is rarely on the oil container. You can only get it by reading the full data sheets.
Group III base stocks are considered synthetic motor oil only in the United States. Elsewhere they are not allowed to be marketed as "synthetic". API (category III) base oils are marketed to the general public legally (in the USA) as fully synthetic motor oil. Within the US, there are no official specifications, or standards as to which oils can be marketed as "synthetic" and which cannot.
Group IV API base stock for synthetic oils -synthetic esters, or PAO - polyalphaolefin. Used in Mobil 1, Amsoils, etc. The first synthetic motor oil sold in the USA with an API rated was by Amsoil.