I assumed this had to do with the reduction in size of shield hitboxes, which I think was overdone.
Used to be shields were a "forcefield" and not only could you not shoot through them, you couldn't walk through them. Now I find that I can use a Heavy Round Shield with 4 shield skill and I'm pretty sure the actual hitbox this generates is only about 80% of the physical size of the shield. Not only do people seem to clip around it easier but I've been hit by projects right through what should have been covered by the edge of my shield.
Basically I think the shield hitbox was over-nerfed and shields no longer even cover the radius dictated by their physical size.
To put it in proper perspective, imagine your juicehead two-hander swiped at someone, and the last foot of your sword clearly intersected their head, but it didn't count because the hitbox of your sword was shorter than the model. That's basically what has happened to shields.