I asked my former priest about this when I was a catechumen several years ago.
He said that he doesn't really know the reason for it, but liturgically we count our days evening to evening (great vespers + Divine Liturgy = 1 liturgical day), but we keep our fasts and feasts on civil time. 12:00 AM to 12:00 A.M.
"Evening to evening" is correct, though I wouldn't define it quite as you do. The Divine Liturgy is not part of the daily cycle, so it should be from the start of one Vespers to the start of the next.
Midnight to midnight fasting is a good rule, too, though I'd rather define it as "bedtime to bedtime" to avoid pigging out at 11:30 - 11:59 pm and then 24 hours and two minutes later feasting again. That's why I also avoid matching fasting to the liturgical day, though if one were truly observant and pious, it would be good to acknowledge in some way the fasting time during the eve preceding the day.
My own priest fasts according to the liturgical day and I know he is of the mindset to avoid the legalism of the 24 hours + 2 minutes scenario. I don't have that strength, so bedtime to bedtime is better.
That being said, I will confess here that my fasting is rarely as strict as it should be because my meals are shared with my non-Orthodox wife. My priest is supportive of this situation.
Long before I became Orthodox, I realized that my body rhythm seemed to want quiet evenings and then be "rarin' to go" in the morning. Yes, I'm one of those obnoxious morning persons. I was thrilled to find that the Orthodox Church sees it my way
, and that was indeed one of the hooks that drew me in long enough to examine the Faith more closely. The whole concept of darkness → light makes so much sense to me.