My wife has anomic aphasia. She can reason and think OK, but she has difficulty in coming up with the words to express herself. In some cases circumlocution works. In other cases, twenty questions works. We also rely on our ability to finish each others sentences.
On the other hand, the calculator and scheduler are pretty much gone. If it is three now, and dinner is at six, the fact that dinner will be three hours from now is a separate fact.
It is the result of a stroke. Small to moderate strokes can result in an array of fairly specific impairments.
Congenital malformations could conceivably do the same, but likely brain plasticity at an early age can compensate for missing tissue in many cases. It might explain certain learning disabilities.
Though yes, as you noted, brain plasticity would likely compensate, at least somewhat. As I've grown older, I've increasingly noticed that there are many people who do seem to have fundamental deficits in organised thinking and linguistic abilities. More often written, but also spoken. Sometimes as elements of another condition (e.g., Parkinsons), but in many cases apparently either congenital or aquired through early environment.
Brain capabilities probably vary as much as other phenotype characteristics. No one without the genes for it will be able to do a good still rings gymnastic routine. And at the other end of performance, many will struggle to finish a 5K. But the fact that you can't run well doesn't mean that you can't shot put or swim well. The use of IQ as a single dimension obscures a multiplicity of abilities.
I've wrestled with (and mostly fought against) single metrics of quality for most of my career, though being able to articulate what the problems are has only come to me relatively recently.
Quality is ultimately suitedness to task, and tasks vary. The more specialised a task, the narrower the quality component parameters, much of which may come from disabling rather than enabling characteristics. Anna Karenina / rocket-science principle: there are many ways to get a complex thing wrong, generally only one narrow path to getting them right.
The breadth of evolutionary adaptations displayed by life on Earth, or even within the narrower scope of Olympic athlete body morphology, is an instructive lesson. None is globally "better", each is adapted (or selected) to purpose or goal.
On the other hand, the calculator and scheduler are pretty much gone. If it is three now, and dinner is at six, the fact that dinner will be three hours from now is a separate fact.
The mind is a collection of apps.