Partly on the endo classification system itself, partly on how people tend to interpret it.
Short text explanation of how endo classification works — Upon observation (usually via laparoscopy), the surgeon makes note of the size and location of any endometriosis lesions they find. Each one gets a certain number of “points,” depending on size and location. (It’s like a game, whee!)
Interpretation mistake number one — Equating Stage IV (severe) endometriosis to more pain and Stages I and II (minimal and mild) to less pain. As many other sources will explain, there is no particular correlation between endo stage and pain. It is quite possible to have Stage III or IV endo and have minimal or no pain or to have Stage I or II endo and have frequent or severe pain.
Interpretation mistake number two — Failing to recognize the strong reproductive bias in the staging system. For endometrial lesions of comparable size, lesions rate anywhere from twice as many to four times as many “points” if they’re on the ovaries as opposed to the peritoneum (pelvic cavity). As for adhesions, the only ones that rate points are those that involve the ovaries or fallopian tubes.
For instance, I have dozens — if not hundreds — of small endometrial implants in my peritoneum, at least some of which (the ones biopsied) are deep. Two points.
There is some posterior cul de sac (basically, the way back of the vagina) endo going on, but not enough for my surgeon to deem it even partially obliterated. Zero points. (Though the gynecologist I saw after I left that surgeon wondered if he hadn’t miscategorized it — so maybe four points?)
And there are also multiple pelvic cavity adhesions — at least some of which, as evidenced by biopsy, have endo involvement. Zero points.
So there’s basically a situation where endometriosis is running deep and wide throughout my pelvis but where the classification system labels it as “minimal” or “mild.” Not only is that a piss poor way of categorizing pain (which, fortunately, most people who are familiar with endo understand), but it’s also a woefully inaccurate way even of classifying the physical impact — the physical presence — of endo on my insides (which, unfortunately, even a lot of people who are familiar with endo do not understand).