Garen's vs Nasus' Q skill

I've been playing Garen and Nasus, and noticed their Q skills (Decisive Strike and Siphoning Strike respectively) seem to be coded a little differently. For Garen, if his Decisive Strike is declared (the animation plays) but the target dies before it hits, he is allowed to retarget his Decisive Strike if time allows. However, for Nasus, if his Siphoning Strike's target dies before it hits, the strike is expended but he does not get any bonus damage, because his target isn't 'killed' by it.

I've already submitted a bug report to the staff, and they told me to post in this forum, as well. I feel like this is a rule inconsistency that needs more clarification.