The Voter Turnouts in Nigeria's Elections do not Add Up.

Can someone explain what is happening here?

Year - Total Votes

1999 - 30,280,052

2003 - 42,018,735

2007 - 35,397,517

2011 - 39,469,484

2015 - 29,432,083

2019 - 28,614,190

2023 - 24,025,940