Surbhi and Rohits deal was revealed later on whereas Deepaks and Rohits deal was shown before the task started.
Surbhi and Deepak both chose Rohit to backstab others. Their reasons can be same :
1) Rohit doesn't have a brain of his and it's easy to manipulate him.
2) If voting tasks comes , he is the most hated and so no HM's would support him.
3) Since Rohit isn't sharp, he might not understand the final task properly and lose.
4) After Deepak only he has that speed and agility in his body and they needed it to remove others from the game.
Now the question arises as why did Rohit choose Surbhi over Deepak ?
1) Does Rohit feel that Surbhi is a weaker competitor than Deepak, so he has higher chances to win .
2) Does he feel after him, Surbhi is also much disliked so there are hopes that HM's might just support him.
3) Does he have a stronger bond with Surbhi .
4) Was he repaying her for fighting for him during the fight she put up with Dipika yo make him safe.
Why do you think Deepak must have chosen Surbhi