Good to hear you've finally received some answers on your child (if this is the same one we've discussed a few times before).
It depends a little on the type of task. At the simplest level, though, yes, it likely does have something to with ASD. I find generally that the cognitive scores of students with ASD don't really stabilize until a bit later than NT students--often well into adolescence. The ability to demonstrate their skills on demand seems to require some executive functions that may be among their lagging skills, and moments of particular rigidity can interfere with optimal performance, especially on timed tasks.
And at the same time, some subtest score fluctuation is also to be expected in the ordinary course of retesting. The reliability of subtests is not anywhere near as high as that of composite/index scores, which is, of course, why the FSIQ is consistent, but the subtests appear less so.
