The CIIE in the Público newspaper:

"The North is not all the same, and the PISA results clarify that.
The explanation for the apparent divergence in the results of northern students may be quite simple: 'The territorial unit is not the same', note Tiago Neves and Gil Nata, researchers at the Center for Research and Intervention in Education of the University of Porto."

Read the full article here: https://www.publico.pt/2023/12/18/sociedade/noticia/norte-nao-igual-resultados-pisa-deixamno-claro-2073655