Some of it is learned through practice, but all of it isn't.
The meaning you intended to convey was probably "not all of it is". Otherwise, the literal meaning contradicts the first part of the sentence. What came up with that phrasing - your conscious or unconscious mind?
I've noticed that the faster I write, the more likely it is my writing will contain homophones. I presume that the faster I write, the more my unconscious mind gets used for the task, and it places more emphasis on sound. Or there's a sound buffer and a letters/word buffer working in parallel, with the former usually taking precedence, but at speed it gets filled too quickly, so the fallback is to the sound buffer.