Leaving heart rate sensor on in battery saving mode improves battery life? Anyone got any idea how or why?

So when I edit my battery saver mode I noticed in wrist heart rate sensor that if I leave it on aka "don't change" selection then it actually adds 9 days supposedly to battery life. Then If I try to set to turn off in battery saver mode it actually decreases it by 9 days. I tested all the other options in battery saver and when you turn something off it's saves battery. On the flip side however, I did temporarily turn off the wrist heart rate sensor while my watch was not in battery saver mode and it add 4 days to the standard estimated battery life left. Any ideas on how it would be possible to gain battery life by leaving the WHR sensor on from  experienced users or garmin staff would be great. Despite its estimates, for now I'm going to keep the WHR sensor off in battery saving mode because it just doesn't make sense to me otherwise. I'm also guessing that one of the answers could just be a minor software bug but of course and corrections are welcomed.