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Humans have been talking for, well, we don't really know, but call it at least 80,000 and maybe as long as two million years.
Not only do we talk, we "learn" to speak despite never receiving sufficient instruction to do so. (This is Chomsky's Poverty of Stimulus argument.) Place an infant with talking humans, and the infant will learn to speak their language -- even if the infant is adopted, and was born to culture speaking a different language. Place an infant among deaf people, and the infant will learn to sign. All this with or without explicit instruction, regardless of malnutrition, destitution, neglect, even if the child in raised in slavery, in prison, in a sweatshop.
Even more remarkably, place infants in a culture where the adults speak an ungrammatical melange made of bits and pieces of several languages -- a trade pidgin -- and the infants, growing into childhood, will invent a new language, with a consistent grammar, known as a creole.
Speech is the most dexterous physical act most of us produce, far more dexterous than anything -- like writing -- that we do with the fingers that distinguish us from animals (though deaf sign probably approaches it): speaking a sentence involves hundreds of co-ordinated movements of the lips, tongue, mouth, and throat, in order to shape a column of air to produce sounds that while unique to each person (this is how we recognize voices) also map, in our listeners' minds, to commonly shared meanings. Even more remarkable, that column of air is produced by overriding and re-patterning the automatic inhalations and exhalations of breathing required to keep ourselves alive.
Contrast all this to writing. Writing is at most 6000 years old; mastering even the rudiments requires years of explicit instruction; and even in the modern West with free public education, illiteracy is hardly unknown. And even the nominally literate are often unable to clearly express themselves in writing, as any perusal of Youtube's comments will attest.
It seems highly likely, then, that composing thoughts into words is an instinctual exercise of speech (in which separate and distinct thoughts are sometimes "jammed" into homonymic "boxes"), and that writing is a taught rather than instinctual conscious process artificially grafted to the end of this naturally evolved chain of processes.
So, it seems that you think in speech, and speech contains homonyms, the correct meanings of which are usually but not always implicit in the context of the whole thought, that is to say, when spoken (and when they're not implicitly clear, a pun, conscious or unconscious, results), and writing is a rather laborious process of General Intelligence as opposed to specialized evolved organs, which serves to transcribe "unspoken speech".
(Reading it seems, is the converse -- I'm reliably informed that less accomplished readers sub-vocalize what they read, and that one can observe their throats tremble as they unconsciously form the words they are reading into speech. Even when I, (an accomplished reader), read, I sometimes "hear" what I'm reading "in my head", as if spoken aloud -- although usually only when fatigued.)
Since applications of General Intelligence are, generally, more difficult than instinct (as brains must be explicitly re-purposed, using taught rather than an evolved algorithms), you're more likely to make mistakes due to distraction or fatigue.
Now, it's a funny sort of mistake: you don't fail to write the word, you just write the wrong homonym. It's as if you are auditing (see the root word there? auditing: hearing) yourself, and committing the wrong homonym to paper. Now, it's not that you don't have a sense of the whole phrase that serves to disambiguate the homonym; you aren't suddenly thing that your though is about a "pomaceous fruit" instead of a "two similar or grouped things".
Or maybe the part of your brain that does the transcription is temporarily "disconnected" from the part that maintains context: perhaps a subroutine has been "cut loose", and so it blindly grabs the homonym that gets the most use. We might expect "pair" to be more often used than "pear", at least for most of us who don't keep orchards, but perhaps there are other factors, even the letters making up the word, or the general words shapes most recently processed. It's unclear.
But what is clear is that writing is supernumerary, artificial, learned and conscious, tacked on to the very end of the long chain of unconscious, essential, instinctual processes involved in turning thoughts into speech sounds (or possibly just making speech sounds, on the not unlikely theory that thought itself is just a strange and baroque elaboration of the more fundamental process of making sounds).
Just as you'll keeping making mistakes in consciously doing sums, you'll make mistakes in writing, even though, absent brain damage, you'll make incredibly few in the unconscious sums you unerringly "calculate" to determine how to move your muscle to walk, or to move a piece of food to your mouth.