There are some countries doing a lot better in reading achievement in primary school than in secondary school according to results found in PIRLS 2006 and in PISA 2006. These countries are for example Russia, Luxembourg, and Germany. In our paper we would like to identify and analyze reasons for this gap. In order to do so our research questions we will take into account the structural organization of the school systems of the respective countries as well as recent reform activities undertaken. Furthermore, for our analysis we will use international data from PIRLS 2006. We assume that the above mentioned aspects as well as information gained from the data can help explaining the gap; but we will also reflect on the question, what aspects cannot be explained by the data used but need to adapt other research methods. Our paper will concentrate on the aspect: achievement explained: inferring cause and effect in the context of cross-sectional surveys, and in particular the challenges inherent in drawing conclusions, given the complexity of educational systems and teaching/learning.