I think it's like this:
The shield reflects perfectly, but big projectiles can intersect with it before they bounce off, and if any part of Naijas body is in the direction of the projectile this will mean the projectile also intersects with that part, causing damage.
So, if you're moving perpendicularly to the projectiles axis, you have the largest chance to not take damage, as no part of your body is close to the edge of the shield. Moving straight towards a projectile pretty much guarantees you get hit, despite the shield.
