Differences in reading measured at school entry are modest compared with the differences visible several years later. The widening is a consequence of how reading is used in schooling.
Vocabulary arrives before school does
Children begin school having heard very different quantities and varieties of language. That exposure shapes vocabulary, sentence structure and familiarity with the way books are organised.
Vocabulary matters for decoding as well as comprehension, since recognising a word is easier when the word is already known and expected in the sentence.
Early screening identifies these differences reliably, but the interval between identification and effective support determines whether the difference persists.
Reading becomes the medium of instruction
In the first years, children learn to read. Afterwards, they read to learn, and instruction across every subject assumes fluent reading as a working tool.
A child still expending effort on decoding has less attention available for the content, so the same lesson yields less. The deficit spreads beyond the reading lesson.
This transition is why gaps that appear stable in early grades begin to widen once the curriculum shifts, without any change in teaching quality.
Practice volume diverges sharply
Fluent readers read more because it is less effortful and more rewarding, and each hour of reading builds vocabulary and background knowledge.
Struggling readers avoid it, so they accumulate far less practice. The difference in exposure grows every year that the pattern continues.
Background knowledge then affects comprehension of new texts, since understanding a passage depends on knowing something about its subject already.
Assessment can obscure the mechanism
Comprehension tests report a single score, which may reflect weak decoding, limited vocabulary, thin background knowledge or difficulty with inference.
Each cause requires a different response, so a score alone is not sufficient to select an intervention. Diagnostic assessment separates the components.
Where systems track only aggregate scores, schools may adopt general strategies that miss the specific difficulty a particular group of pupils has.
Timing determines what support achieves
Intensive support delivered early, in small groups and by trained staff, has the strongest evidence base. The same support delivered later addresses a wider gap with less time remaining.
Delivery depends on staffing and timetabling, so schools with tighter budgets deliver less of it, which links the response to the funding patterns described elsewhere.
Curriculum requirements, screening obligations and funding for reading support are set nationally or regionally and are revised regularly, so provision differs considerably between systems.