That has probably something to do with the ground mass.
Under Character Editor -> Physics, reduce the value for the ground mass from 2 to 1 or less.
To visualize that collider, navigate to your character prefab (where the Hit Boxes script is located) and toggle "Preview All Boxes":

On a side note, as for the disparity between the hitboxes and the animation, due to the use of certain rotation options options, some animations might not perfectly reflect how the animation maps are being calculated. To fix this, untoggle "Use Animation Maps" under Character Editor -> Move Sets.