Slow-but-accurate writing speed would definitely be a plausible explanation for lower subtest scores in Alphabet Writing Fluency, both Math Fluency subtests, and even, to some extent, Sentence Composition, as he may have been investing some energy into correct letter formation that could have been devoted to sentence formulation. If there is a marked difference between AWF and SC, that would more clearly support the impact of writing speed on his overall Written Expression Composite.
So far, I get the sense that all of the fine-motor tasks are age-appropriate for speed and accuracy, so I'm not ringing alarm bells (especially in a young boy in Montessori), but I'd definitely keep an eye on handwriting speed and automaticity over the next few years. If it doesn't appear to keep pace with NT peers, there might be more to it. Also, even if it's just asynchrony, when advocating for more challenge in areas that require some kind of handwriting response (language arts, spelling, sometimes math, etc.), you may find that accommodations and scaffolding for the physical act of writing may be necessary to give him access to the higher level skills. For instance, if his appropriate instructional zone for reading and comprehending is third grade-level text, but his physical writing skills are on grade-level for his age, teachers may need to assess his comprehension (let's say in the form of a book report) through alternative tasks (e.g., oral/dramatic presentation), or assistive technology (scribed report).
