Fair points, polarbear.
In my DD's case, I tend to be led by my own experience, primarily because DD has demonstrated time and time again that the way she learns is identical to my own processes. Because she has parents who are far more involved, and she lives in a much more stimulating environment, she hits mental milestones well ahead of where I did. So, anywhere that she's behind where I was at the same age, it stands out... and there's usually a reason behind it.
The only other time something like this caught my attention, she had just turned five, and was still not reading an entire (age-appropriate) book aloud, or silently to herself. DW and I knew she was capable, because she'd read words/short phrases on signs or menus, some of them pretty impressive. I appealed to her pride... "You're ahead of me in every way, except one... I was reading aloud to my mom when I was 4." She read aloud to her mom within the week. By the time a month had passed, she was reading daily to her pre-K class. That clinched it... there was no issue of ability at play, just perfectionism and fear of failure, which she finally overcame.
So, with that in mind... when I was 8, I won a district award for an essay. I still remember what it was about. And I remember that I didn't have any problems with spelling.
In DD's case, what's different is the amount of written output beginning in K. We did not journal every day, beginning in K. We wrote letters, then words, then simple sentences, and then paragraphs, in K-2. They were graded for grammar and spelling. We progressed to short stories in 3rd.
As I've said, DD learns the same way I do, which has caused me to joke that I have the user's manual to her brain (she says, "That's creepy"). The act of writing something down practically inscribes it into my brain (for instance, I find great value in taking notes, and then never again looking at them). In her place, if I were writing using inventive spelling, I would be inscribing that spelling into my brain... and it takes far more effort to unlearn an incorrect thing than to learn it correctly in the first place. This method would be toxic to me. In her output, I'm seeing exactly what I would expect to see, had I been put in her place.
In her place, I would be ANGRY, because something that should have been easy, had it been taught correctly, will now be something difficult. At some point, someone is going to hold her accountable for spelling correctly. And given her perfectionist personality, that likely won't go well. I would really hate to see her decide she's a bad writer based on this, because in every way except spelling, she's a natural.