Probably rule of cool and artist error. Sometimes the shield is held upside down but for very niche situations, such as you are walking out from under a patio and a threat may be on the level above you, so you turn the shield upside down and held above you so you are covered and can shoot, but your shield does not overextend in front of you which is awkward to hold and prematurely gives away your presence.
Most modern ballistic shields have a horizontal rather than the vertical handle hold that is shown here, and the horizontal tends to be superior as you can maneuver the shield in pretty much every position without having to turn it upside down and without strain on your wrist trying to keep it from sliding around your forearm. For example when clearing a second floor you can instead hold it above you parallel to the ceilings edge, treating that edge like a corner and "pie" it.
It also makes it easy to drop the shield to slung for things like reloads or medical treatment as you just drop your arm down, slide it up and out of the sling, and then when you want the shield again you drop your hand back in.
