When I was typed at 6, telling myself that descriptions were bad was just a part of my denial. Yet, I was surprised by how many other 6s came forward to say that they couldn't initially see themselves as 6s, either. It seems like a relatively common phenomenon to me, based on this.
I obtained a copy of Wisdom of the Enneagram about a month ago (which tends to be the most widely-read description), and I can see why mistypes might happen. Their depiction of the type sounds, in places, more like an anxiety disorder, and they emphasize inter-relational security (rather than wanting to find a solid foundation in life) and a fear of "trying new things"--which might stand for an SJ 6, but an ENTP 6w5, for example, might be a loner enthused at ideas of diversity and a brighter future (my father being one such). They also state that 6s are always aware of their anxieties, yet many cp6s actually think they are angry rather than anxious.
I've likewise met 1-fixed 6s who mistype at 8 due to the ethos of "stand your ground, look after yourself, trust no one, and struggle hard for your existence". In Wisdom, anyway, the open self-interest and even sadistic qualities aren't especially emphasized. I can see why there would be many mistypes.
It is possible that the 6s I spoke with were mistyped, and also possible that other types fail to see themselves in their respective types at a similar frequency to 6s, of course. I certainly have my share of gripes with the 4 description in Wisdom.
Just to be semi-constructive here, I find that the works of Tom Condon, Helen Palmer, Naranjo, as well as the descriptions on Ocean Moonshine to be very accurate in portraying the core psychology of the types. Other descriptions aren't necessarily as accurate--I saw one site telling me that 6w7s were all Republicans and 6w5s were all Democrats, for instance. This can be confusing for a casual surfer.
I think this is actually a fair point that no one seems to mention. Eights are a reactive type and express anger openly. They are often less inclined to see threats than a cp6 (or at least, perceive threats differently) and can be less volatile in certain respects--maybe roll more things off their back--but there is also an extremely low threshold for frustration and annoyance.
You've articulated it far better than I ever could, so I'll leave it here, but I think you are right to divert our collective attention back to that fact.