Okay...I'm back. Whew!
Cnm, there typically are not extended norms for achievement tests, as there is a point at which a more useful measure becomes criterion-referenced (vs age/grade-norm-referenced, which is what most achievement tests reported here are), so that one may determine a true instructional level.
But let's imagine there were extended norms. A 160 on both letter/word and reading comp does generate a small range of ways to reach 160 on the composite, but it's only about seven points of room. More importantly, the subtests each have about 30 raw score points (out of about 100 in each case) range that results in the same 160 max standard scores. So yes, these reading scores may not fully capture your child's reading skills, but the ceiling would hit at the subtest level, and would be visible in the raw scores.
On a side note, did they report those scores as age- or grade-based, and, if grade, were these k or grade 1 norms? (I'm assuming kindergarten or age-norms at the moment.)
And fwiw, the math and writing scores may be somewhat more affected by instructional exposure, since some math requires knowledge of the arbitrary conventions (what specific symbols represent), as does writing (e.g., punctuation and writing formats, such as how to address an envelope or write greetings in a letter).
