Isn't there a dissimilarity in the collider size for a melee weapon
vs a projectile ingame and IRL.
For instance a club collider would be shaped like the inner perimeter
of the object giving it a greater chance, to contact more colliders
or surface area. While a projectile has a diminished collider size
and a single point trajectory vs the angular sweep.
The visual I have is of a a bullet, and a club both hitting a
shatter resistant piece of glass. The bullet impact area is a point,
and the club, depending on the angle fractures an area.
I-frames don't have to be intentional, it could be a simple disconnect between
transitions of a state engine, or since most representations of colliders I've seen
are planes. So conceivably if a zombie is at the correct angle during impact, you
are only presented the edge as a target, vs the normal surface area.
like a paper doll turned side ways