It appears that for some people (or driving habits), their logic was flawed and the reduced state of charge is resulting in the early death of batteries. Instead of maintaining a true float voltage, manufacturers choose to allow the battery to stay in a less-than-fully-topped-off state to eek out a tiny bit more gas mileage. Every modern car that I've heard of will automatically control the alternator output to maintain voltage only as high as needed to keep the battery's state of charge at an acceptable level.
