When the A.I is running at the object and it gets shot as soon as it touches the object there is a chance that it might be launched up into the air (it might not be possible if it isn't jumping before it touches the object) and the A.I takes no damage when it hits the ground. This same kind of phenomenon is also observed in Halo: Reach. Here it is mostly Covenant Unggoy and Kig-Yar during Firefight or Campaign.