I have noticed the same, that there are a lot of different things that people popularly label "patchouli". Some of the "hippie" patchoulis and patchouli perfumes (not essential oils) I think just have more things in them than just patchouli, but that people often don't/can't identify what's the patchouli and what's the "other stuff". Some smell more incensey to me, some more soft and powdery, and I think it's just because the patchouli in them is like eggs in cake: yeah, all cakes have eggs, but they're all different due to what else is in them (and you'd not point to a cake as an example of an egg...this metaphor might not be the best, but I think you get my point.)

I do imagine that there are different grades, types, species, and preparations of even pure patchouli oil. Different varieties of the plant, where they're grown, how they're distilled, etc. This page seems to have a nice video that describes how Indonesian patchouli oil has varied types/grades, and they do say they smell different, and how the industry eventually shifted to use the cheaper versions. All I could suggest is that you try to find some reputable places that will sell you isolated samples of JUST different types of patchouli oil (rather than patchouli FRAGRANCE oils which are generally just stuff made to 'smell like' patchouli), so you can give them a sniff. Just be careful, though, as some essential oils are very concentrated and can cause some skin reactions undiluted (and I can't remember if patchouli is one of them).