The strategic (non-)implementation of EU rules is an important topic in the EU literature. During the European migration crisis, several member states have deviated from the clear rules set out by the legal framework of Schengen and Dublin. While the political controversies around these deviations have attracted much attention, member states' national authorities also have some implementation leeway in everyday decision-making. Little is known about this leeway and whether, how, and why member states use it. The Dublin system allows countries to send asylum seekers in an "outgoing procedure" when they conclude that another Dublin state is responsible for the person in question. Focusing on this administrative decision, I argue that for efficiency reasons, national authorities specialize in asylum seekers from countries of which they already have a large share of residents. Asylum seekers from these countries are less likely to be sent in an "outgoing procedure". Empirically, the probability of such a procedure for an applicant is calculated with a multilevel model, using register panel data from Switzerland. The preliminary results support the main arguments. By shedding light on how the strategic implementation of rules may lead to imbalances in the distribution of asylum seekers, these findings have broader implications for the Dublin system.