If he really doesn't want a free ride and is wanting to go up, then why didn't he just ask everyone to nom him, like Marcus did in the past, thus ensure a Marcus vs Siavash eviction. He told Charlie that he didn't want this because it would be Marcus vs Siavash. Why not this scenario then?
Going by what Siavash CALCULATED IN HIS MIND, not nomming David ensured a likely 3-way, putting himself up with Marcus and David. This scenario seems fine to him! By doing this, both himself and David would have 3 noms each. It turned out he made an error RE Rodrigo in the end.
My only logical conclusion is that he's happy with a likely 3-way eviction, but NOT happy with a 2-way eviction. LOOK HEROIC FOR NOT NOMINATING THUS PUTTING HIMSELF UP + 3-WAY EVICTION WITH UNPOPULAR DAVID = TOP RESULT! 
He admitted a risk of a 2-way eviction, it was a risk he was willing to take. Playing the 'heroic' card. Also he would look BAD if he tactically tried to ensure a David vs Marcus eviction; contradicting his 'I want to be up, 'No Free Rides', 'Anti-Scheming' ethos. So this wasn't a good option RE public image!
Why else not ENSURE a 2-way eviction by asking others to nom him like Marcus had done the past 3 weeks, if he wants to go up THAT much?
FULL SCHEMING EXPLANATION: thisisbigbrother.com/forum/viewthread.php?tid=143725
Evict Siavash this Tuesday! Charlie/Lisa votes will split and he could go! 