Terri winning was rigged.
The housemates could evict someone on day one. They evicted her. BB didn't want her gone, so they gave her a task in the house to earn her place as a housemate again. I think it involved that party boy Corey. She failed the task. Big Brother let her become a house mate anyway.
On the final eviction night they said they would not show the faces, just the percentages. Terri wasn't winning. So they then showed the faces next to the percentages. With Ben winning. They said they wouldn't show the graph again. They were saying stuff like, 'If you want Nanna Terri to win, remember to vote!'
They then extended the voting period, running the show longer. Then they showed the graph again with Ben still winning. They then appealed to the audience to vote for Terri if they want her to win. Then once she was winning, they closed the voting lines and she won.