Answer:
Voter turnout is a figure which is used to find out the total number of people eligible to vote and also helps us find out how many people have voted in a particular election. On the basis of society, many citizens don't vote and many citizens leave the country and stay in other countries as immigrants, due to which the number of voters per election decrease in number. This is the reason why the voters turnout vary from epection to election.
This is just my opinion, you can make changes to it if you want.