There was an alliance of 6 and an alliance of 3
Mike won immunity so he could not be voted for.
The alliance of 6 decided to split their votes 3 for Jenn and 3 for Shirin so that if a hidden immunity idol was played for either then the other would be sent home.
Survivor rules state that in the event of a tie (if the alliance of 3 voted for the same person and the alliance of 6 split their votes 3 and 3) a revote happens where you can only vote for the tied players and the tied players do not get to participate in the revote.
A three way tie would cause a revote with Shirin, Jenn and whomever the alliance of 3 three voted for not being able to vote (which would make it 5 votes vs mike 1 vote)
It was probably planned within the alliance of 3 for them to add their two votes to Jenn so she would go home and Shirin would be safe. Jenn has been saying this episode and last weeks episode that she is really not happy and really doesn't want to play anymore. Last night's episode she said that if she were to continue on it would only be so she could mess up the alliance of 6 game (like whispering and accusing Will of having more food than what he shared)