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TheMixed Member Proportional representation (MMP / AMS) tries to link two (partially opposed) principles:

• Elect the candidate for direct election nominated in the constituency.
• Proportional Party Representation.

But this only works, as long as the party wins more seats by the party vote (secondary vote) than by the first vote (constituency vote).

If a Party is awardedless seats, as it needs to justify the constituency seats won, the missing proportional seats are additionaly awarded as additionaloverhang seats (see § 6 Abs. 5 and § 7 Abs. 3 sentence 2BWahlG).

All other conceivable reasons cause overhang seats by changing reason one or two:

• Cast with the constituency vote a candidate of another party as casted with the party vote "Splitting of votes".
  Thus, the electors can target both reasons.
• Poor Polling A small election turnout in a country (Bundesland, federal state) reduces the number of party votes (Reason 2) of the Land's List.
• Several (Three or more) relatively strong parties (week Constituency Winners) with narrow majorities.
  Especially in the new eastern countries, where there are three bigger competeting parties (CDU, PDS and SPD) with chances to win constituencies. The strongest party can win almost all of the constituencies (reason 1) with close votes (reason 2).
• Distribution of constituencies on the lands
  The number of constituencies in a land (federal state) is the maximum number of constituency seats a party can achieve there (reason 1). In the 1998 election four overhang seats were caused by an unproportional distribution of constituencies (WP 96/98). (3 in 1994).
• Gerrymandering (Zoning of constituencies)
  It seems this is the only point of interest for the parties and the Bundestag's reform commission, although it is rather irrelevant in comparison to all other points. If all the constituencies are too big, it would touch their distribution into the countries (land's). With gerrymandering, one could influence the winner in same constituencies (and with that, reason 1) but practically, this seems to be really difficult.
  Many won constituencies of one party (and thus overhang seats) are favoured by more morehomogenous constituencies, because the conditions to win one constituency are the same as to win all constituencies. On the other hand, overhang seats are more unlikely in aheterogenous set of constituencies, because the conditions to win a constituency change from constituency to constituency. To win almost all of them, means to reach more than 50% of the votes, by any means.
• Effect of subdivison by the Hamilton's method.
  Because Hamilton's method (Hare/Niemeyer) is not consistent, the number of votes for one land's list affect the allocation of seats between two others.
  E.g. in 1994 additional votes for the SPD in North-Rhine/Westphalia would have cost the SPD would have cost an overhang seat
  In extreme cases this could mean that a land's list which wins votes receives less seats. (Example).