Simplify the following text for a Grade 9 native English-speaking person.
Original: Parties that obtain fewer constituency seats than their national share of the vote are allotted seats from party lists to make up the difference . In contrast , parties that obtain more constituency seats than their national share of the vote are allowed to keep these so-called overhang seats .
Simplified: Sometimes a party already has more constituency seats in a land ( state ) than it should have to keep the percentage of votes and seats equal .