Just taking a wild guess here ... but I'd say he's spelling this way because he's used to reading phonetically? As opposed to if he was a sight reader, he'd know how to spell the word because he'd have the whole word memorized? Like I said. Just guessing here.
I can confirm it doesn't work that way for my sight reader. Spelling is its own thing. And his reading guesses are fantastical (good and bad that way.) I'd say for a sight reader who uses pattern matching it is more like I can smell a carrot cake baking, but I couldn't tell you all the ingredients, their quantities, or put them together in the right order, etc.
We needed writing samples for DS7, but couldn't even guess where he was. I couldn't find an online resource, nothing. But when I walked down the hall at his school and saw the stuff posted, I was pretty confident of where he was in a short period of time and what the next level skill looked like.
The big thing I learned in exploring that topic, is a single sentence isn't enough to answer the question. The common core has complexity using a picture and words to explain it. Kindergarten definitely included concepts at a paragraph level as a standard. I think spelling is a different standard.
