I agree, it's probably as Sony intended.This is working as designed. For accuracy of image, you set it to Medium. If you care more about blooming, you want it set to High.
I would also add that Medium is the setting you want to choose for the brightest HDR. If you have letterbox bars on a movie and notice too much flashlighting from the corners of your tv, or the bars seem too gray, then that might be the one time to go ahead and use High (but I usually don't).