If you're referring to the few-point difference between 2012 and 2018 in the first chart here: https://www.oecd.org/en/publications/pisa-2025-results-volum... I think that's more likely to be statistical noise. Keep in mind that PISA was originally normed to a mean of 500 and standard deviation 100. Even the more obvious drop after 2018 (I blame the pandemic) down to a mean of 466 on the reading test in 2025 isn't all that huge in absolute terms, though definitely statistically significant with the large sample they have.