Summoned WitherSkulls and SmallFireballs Move Jerkily.

I was playing around with entity summoning today when I discovered that WitherSkulls and SmallFireballs can now be /summoned into the world. Like /summoned Ghast Fireballs, their movement quickly decreased in the air but they would explode upon impact if moving fast enough. Unlike Ghast Fireballs, though, they would only render about once each second, so the movement appeared choppy.

To reproduce:
1) Go into creative world
2) Fly up a bit and face the -x direction
3) Run: /summon WitherSkull ~-8 ~ ~

{direction:[0.4,,]}

EDIT: After some testing I found that the movement of these projectiles is related to the "power" tag somehow. Setting "power" to just the right values will make the projectile move smoothly. Why is this necessary if it's not necessary for big Fireballs?