Germany’s mixed-member proportional system, internationally known as the ‘German model’, has been recently criticized, since the 1994 Bundestag elections saw the genesis of several surplus mandates giving the CDU-FDP-government a safer majority in parliament. Due to this institutional effect, a lot of political scientists and lawyers have argued that the electoral system is no longer in conformity with the constitutional principle of electoral equality. From the perspective of empirical political analysis, two important aspects have not been taken into account in this debate. First, the complex relationship between different factors causing surplus mandates have not been analysed in a systematic-comparative manner. Second, the literature does not explain why so many surplus mandates have appeared in all national elections since 1990 whereas almost none were produced in Bundestag elections before the reunification. This study analyses the genesis of surplus mandates having appeared in the three German Bundestag elections since reunification (1990–1998). In contrast to monocausal explanations it confirms the hypothesis that in the present political context almost all surplus mandates result from complex relationships between generally known explanatory variables. Furthermore, the empirical analysis shows that the genesis of surplus mandates is not only caused by particularities of electoral districting in certain Bundesländer, but also by politico-structural differences between East and West Germany.