Because the votes for Leorio are primarily independent.

Pariston's speech will create a split among the independents, who were all most likely leaning toward Leorio.

It doesn't matter what you say. My eighty-seven votes, Mizai's seventy-two votes, and the fifty-seven independent votes will be added to Leorio's ninety-five.

When everything comes down to swing votes...

at a school with over three thousand students, but the idea of creating a ballot box never occurred to us.