People with atypical development tend to almost inevitably get higher IQ scores as they age, given a good environment and appropriate education.
That is because IQ tests are normed to neurotypical development.
Okay, imagine a line graph, with developmental milestones on one axis, and age on the other axis; and have NT development as a straight diagonal line--typical kids, getting their skills at the typical rate. That's the "100 IQ" that you're going to be comparing the atypical kids to. Now take a gifted NT kid. His line is more steep because his development is faster; he does things earlier. The slope of a delayed NT kid doesn't rise as fast. But both the gifted and delayed kids will have straight lines, so that the ratio of where they are compared to the average kid can be used to denote their IQs.
Not so much with autism. Our developmental lines are curved, not straight. Sometimes we develop faster than the NT average kid; sometimes slower. Sometimes the line even goes down instead of up, and they call that "regression" (and give a diagnosis of CDD, in the obvious cases). Sometimes there's a plateau, keeping the same skills for a while. Sometimes we get it all at once, and there's a vertical jump in the line. Autistic development is all over the board, sometimes unpredictable, sometimes just unusual. If you measure an autistic child's IQ, you're assuming that he must have a straight-line developmental path, like a neurotypical child does; but he almost certainly doesn't. For that matter, he probably has separate developmental paths for separate skills; he may be way below average for one skill, way above for another, average in a third; catching up in one while falling behind in another. That is very normal for someone with autism. Quite simply, we don't grow the same way non-autistic people do and our IQs aren't static because they're measured in comparison to people who don't develop the same way we do.
BTW, while I'm referring to kids because it's kids' development that's most obvious, this does continue into adulthood. Even NT adults don't stop developing when they hit 18; autistic ones even more so.