Your body position is much different. Your are more "on the gun" while prone and "behind" while benching. Try sitting higher on the bench seat, you are almost leaning over it more so and getting closer to mimicking the prone. Problem usually is bench height and seat level, It's been years since I shot from a bench, I used to sit on one leg to raise my height.
Yeah, heavy calibers are not fun prone, I mean heavy ones like mentioned. When I check my slugs, I use sticks or barricade.