Jenna/Heidi are only allied with Rob out of necessity.When Christy let the power trip go to her head she made Rob very nervous. Rob couldn't risk that Christy might actually vote with the girls against him.
So Rob gave Jenna/Heidi the one thing they wanted more than voting him off... that's voting Christy off.
Heidi and Jenna had no guarantee that Christy would vote with them, and facing the prospect they could find themselves on the wrong end of a 4-2 vote, they jumped at the oppurtunity to save themselves and get rid of the one person they like less than Rob.
Just as a drowning man will grasp at straws without any reasonable expectation that it will keep them afloat, Jenna/Heidi grasped at the only chance they had to stay in the game, voting out Christy.
I don't believe Jenna & Heidi trust Rob at all.
After Rob was able to get them drunk and show off his speedos the girls could see with their own eyes how much Rob still liked them...Actually I think for the same reason that people are still holding onto the ChillOne as being valid, there is nothing else out there for them. Voting for Christie would have absolutely no impact on the downside for them, and the upside potential is huge.
"Give me chastity and continence, but not yet."
- Saint Augustine (354-430)