Personally I think IQ and practical intelligence differs for a person. I have know many IQ smart people that can solve complex problems but fail to solve or foresee everyday basic problems
That being said, the caveat "on average" must always be added. On average, on average, on average. Same goes with anything about average tendencies of different genders. On average, on average, on average, averages across large groups do not map to individuals in that group. Emboss that on a mallet and bash people over the head with it until they get it.
I do wonder if there might be some conservation at play in some individual cases -- which kinda matches what you said. There is only so much brain tissue. If you're hyper-good at some narrow kind of analytical intelligence (coding!) maybe some of that brain tissue is borrowed from other things it might be doing like understanding other meat sacks.
But this is biology after all. Most likely, plain improvements with zero downsides, very narrow improvements at the expense of something else, and improvements at the expense of life expectancy, are all possible mutations.
Yeah when you get into the upper tail it starts to get hard to find things to differentiate people.