This Sivir is meant to take advantage of her 'Q' that has the longest range out of her attacks.
Since 'Q' scales on both Attack damage and Ability Power, the way my items are set up helps maximize that damage. The reason for the runes are because you took the 15% magic penetration from masteries and you are also taking 40% magic penetration from void staff. Because of how Riot does their diminished returns for % magic penetration, you end up with 49.99% magic pen + the runes. (About 8.5 magic pen)

Item and Additional Thoughts
Up to Void Staff, everything is mandatory. Infinity Edge is suggested for its additional damage when Sivir is autoattacking with her 'W'. Bloodthirster is for additional damage as long as you keep alive, plus the fact that Sivir is the 2nd Goddess of farming (behind the bird). 40 minion kills can be obtained very quickly. For the 6th item, it really depends.
If the game is really dependent on map control, I usually do not buy a 6th item and rather focus on wards, and take the red,blue,green pots constantly, pink if you need it. If your teammates are doing their jobs, and you the carry can use the 6th space, there are 3 choices for the 6th item.
1. Banshee's Veil (By far the most safe often + her 'E')
2. Black Cleaver (They take the damage from somewhere, her autoattacks or her 'Q', the armor debuff really helps kill the other carry)
3. Another Bloodthirster (Want more damage and you arent dying a lot? Grats go kill them even more often
After the 6th item, the normal red,blue,green pot since Sivir scales off all of them.