In the frame of national Italian standardized assessments in mathematics, a didactic phenomenon has been observed concerning students' behaviour when answering an item: students are inclined towards not accepting an output which is not clearly recognizable with something distinct from the starting inputs. The observed effect seems to originate from the teacher-student relationship and could be described as a typical didactic contract situation but it also affects the general relationship between the student and mathematics. Adopting a mixed-method apparatus including large-scale assessments results, questionnaires and interviews, we can describe several behavioural features related to the observed phenomenon. In particular, we show how it is present in different grades and in pupils of different abilities. We consider the 'ruling principle' as a connecting concept for the different constructs used here to analyse this phenomenon, i.e. an organizational or normative criterion that shapes the action of the student facing a mathematical task.