Why does Twitch with Runaan Apply Poison but Vayne doesn't apply silver bolts?

If poison coated arrows come from twitch's crossbow Then aren't silver bolts coming out of vayne's crossbow? I smell black magics
Proposed Change:

--Silver Runaan-- Change: Runaan now applies Silver Bolts to all targets. imprudent fools tumbles in vayne "We've considered that Twitch applying his venom but vayne not applying silver bolts was not fair, after all the poison arrow come from the same crossbow and vayne's bolts come from the same crossbow".

http://orig05.deviantart.net/48e1/f/2015/346/c/c/_lol__vayne_tumble_gif_by_augishd-d9jw1sk.gif