I have read lots of slave narratives and agree with your characterization. They are very interesting and somewhat different than I expected. One possible complication is that the project happened 70 years after emancipation so it might be that slaves who had harsher owners were less likely to survive to be interviewed. Also most of the people interviewed were children during slavery and may have been somewhat shielded by their parents.
On top of this, people tend to remember their childhoods and youth fondly so there is that aspect as well. Even with all of that said though, I still find myself weirded out…like I said, not sure what to make of it.
It's not so surprising to me. It would be more surprising to me if e.g., slaves didn't think slavery was kind of an alright thing. If the alternatives are to come to terms with your conditions or die, then, well... all you're gonna have left is the people who came to terms with it. And of course, the slavers are going to be modifying themselves as well. The slaver who ends up having to kill all his slaves because he treats them so poorly that all of them find it unbearable, he isn't going to be operating for long. So we have these two forces pushing people towards turning slavery into a... sustainable... institution.
Everyone develops some sort of sense of a proper state of the world during development. How else would people of the past have found things tolerable that we would never put up with today? Did they just sit there seething with rage 24/7? No. They thought the way things were was the way things should be. Even when the revolutionaries were beheading their kings, the kings really had to drag them to that conclusion kicking and screaming the whole way by being massive dickheads about it. Almost none of the people who ended up abolishing their monarchy set out with that as their goal. And, just as notable, the kings themselves found the concessions they were having imposed upon them intolerable enough that they chose death instead. These were people who were very different from us in what they thought was the proper state of the world. It would be weird if people of the past wanted their world to be like the way we want it to be.
I also think that something similar happens to tolerance for pain and other discomfort. When I see the things people in the media complain about, I often think of The Princess of the Pea, who could feel that she was sleeping on a pea even through a hundred mattresses. This was a sign that she was true nobility! No mere peasant could fake such sensitivity to discomfort! They would not have been able to feel it at all.
Personally, I spent a lot of time in hospital as a child, enduring a lot of pain and discomfort that really there was nothing to do anything about. On one hand, it made me very tolerant of pain, but on the other hand it also gave me a sort of feeling of helplessness towards it. Whenever I feel pain or discomfort now, I by default just sort of ignore it, even if I can prevent it, because it doesn't naturally occur to me that something can be done about it. Which almost lost me a toe to infection at one point, because the wound didn't bother me, so it didn't register with me that it not healing should've been alarming. It was just another thing that was happening that wasn't important enough to pay attention to. So now I consciously pay attention to any damage my body gets. It also feel like I'm not able to empathize at all, with many people who suffer. I just look at them going through their agonized performance and feel nothing. I'm reminded of reading about how psychopaths are themselves less sensitive to pain, and how this may explain how they're insensitive to pain in others. They simply look at someone suffering and feel nothing, because they would feel nothing in that person's place. I'm not that extreme, but I do feel bad about my insensitivity sometimes.
Anyway, I imagine these ex-slaves must've felt something similar. Their toughness, derived from personal suffering, they may not even have felt that e.g., being beaten by their master was an especially painful thing. It's just sort of something that happens, can't do anything about it anyway (and you probably did something to deserve it!), and then you move on with your day. The worst thing about it that the wound keeps aching as you go about your chores afterwards.