8 on picture completion is probably low-normal. That's the subtest where you have to figure out the "what's wrong with this picture" cards, right? If so, it seems like they are probably checking how good you are at making a big picture out of details and then reasoning which details are missing.
The picture arrangement thing has, I think, some card series that have socially oriented stories and others that don't. It may depend on which cards you're better at arranging. I had the same thing on my report; I had trouble with socially-oriented picture arrangement too.
It may just be that your picture-arrangement score was a lot lower than your score on similar activities without a social focus. You can have a specific problem with an average score, if all your other scores in that area are above average.
How'd you do on matrix reasoning? I always ask about that because it's a very visual/logical task and I've seen unusually high and unusually low scores on the Spectrum. It seems to be a good indicator whether you're a visual or verbal learner. My score on that one was unusually low, so I'm definitely a verbal>visual kind of thinker.