An acute-on-chronic bleeding is a combination of hyperdense and hypodense, which is seen in individuals who have a rebleed in the bridging veins after a chronic hematoma has already formed.
So, brain herniation can be caused by either a focal mass effect, like the arterial bleed, or a diffuse mass effect, depending on whether the problem is in one area or involves the entire brain.
Top to toe, buggy hematoma on the side of her head, chest wall bruising from the seatbelt, got laceration with arterial bleed on her leg, RPC stable, GCS is 12. - This is literally not how this should happen.