This is a sentence from Wikipedia.
Perilla oil is an edible vegetable oil derived from perilla seeds.
I don't understand why it is an edible vegetable oil. We can only have one oil from perilla, i.e., perilla oil. So, it should be the edible vegetable oil, not an edible vegetable oil.
If the latter part derived from perilla seeds of the sentence is removed, it makes sense. We say Perilla oil is an edible vegetable oil. There are many edible vegetable oils, and perilla oil is one of them.
However, things change after adding the latter part. And the noun oil should follow the definite article the since we can only have one oil from perilla.
Any explanation to it is welcome.