In today's information age, developing data science competencies has become vital to fostering responsible citizenry. However, the actual techniques learners need to become proficient in are still somewhat "in-construction", as the relatively new field of data science is constantly expanding to meet new data-related demands. Data science education needs to develop innovative means to keep up with this expansion that focus less on proficiency in specific techniques, but rather introduce novices to authentic data practices, and the authentic purposes directing the authentic practices. This paper focuses on a specific practice, the use of simulations to generate and examine data, in the context of authentic scientific Citizen Science research. We provide a case study of one pair of middle school students' engagement in an extended learning sequence including simulation activities inspired by authentic data practices, adapted to also be authentic for young students. While the simulation activity was inspired by the scientists' purposes, our findings illustrate four different actual purposes the students attributed to it. We also show that as the students deepened their engagement with the simulation, they gradually appropriated its intended purpose, alongside articulating more mature views of data-related concepts. The conclusions summarize the four different purposes the students expressed and identify aspects of design that contributed to the gradual re-shaping process of their actual purposes.