Hi Mila,
Thanks for voicing what can be a confusing or even frustrating pattern for teachers to see: an ability for students to seemingly be able to perform an action or skill in one setting or context but not another. It’s not uncommon for even well-educated adults to have words that have remained difficult for us to spell over the years. One of mine is restaurant. I’m not sure why I still have yet to master which vowel is spelled with an <a> and which with an <au>, but I haven’t. When I slow down and think through my spelling pronunciation of the word (saying “rest-AU-rant” silently to myself), I can (usually...) assign the medial vowel the <au> grapheme. I usually have more (important?) things I am also thinking about when writing that word out, and I often don’t bother to stop my train of thought and think through the spelling of the word, assuming that it will be a close enough approximation for either the reader to understand or spellcheck to catch for me.
Young students, however, are likely to have this experience with more than an odd word here or there. And the cognitive effort of thinking through the spelling of so many words is likely to really come at the detriment of formulating coherent thoughts and organized writing. Add to this that they may also be thinking through the correct size, orientation, and formation of each letter or searching the keyboard for the correct letter selection. They may not have yet internalized spacing between words or punctuation and capitalization conventions. They may also be relying on graphic organizers and referring back to notes steering the direction and organization of the piece. All this to say, inexperienced writers are often juggling a lot of demands, and sometimes some balls get dropped.
This is, in essence, a driving force behind the inclusion of structured, repetitive practice of skills in isolation before gradually increasing the demands. The theory is that if a lower-order skill is practiced to the point of automaticity, it won’t bog down a person’s cognitive load, allowing them to concentrate on higher-order skills such as writing. This practice applies to tasks such as letter formation, keyboarding, decoding, and, yes, spelling.
Dictation tasks, whether at the isolated word or sentence-level, provide much more support and structure than authentic writing. During dictation, the student already knows which word they are supposed to spell and can devote nearly all of their attention to encoding it. Dictation omits the necessity of simultaneously generating ideas, organizing sentences, retrieving vocabulary, monitoring grammar and punctuation, and maintaining the overall meaning of the piece. A student may absolutely know how to spell a word in isolation but still fail to retrieve or apply that knowledge consistently during connected writing.
This is why transfer often requires more than simply demonstrating mastery during isolated practice. Students benefit from explicit opportunities to apply spelling patterns during increasingly complex writing tasks, while still being provided frequent- if not immediate- corrective feedback to support their learning. Over time, repeated successful retrieval in authentic writing contexts helps these spelling patterns become more automatic and durable.

