Thank you Dao. It is interesting topic. Again when we do the math it doesn't make sense, though. Will give a simple example.
When the Jupiter, Moon and Sun conjunct in Zi constellation the baZi Would be:
Jia Jia Jia Jia
Zi Zi Zi Zi
Again as so much of these relation its really poetic and it seems so logical if we don't go in depth.
However it doesn't take much knowledge about the Solar Calendar to figure out that if the year is Jia/Zi the month branch Zi will always come with Bing stem...
So Jia/Zi year and Jia/Zi month can never happen in the way the calendar is calculated...
Basically the stems of Year and Day are more free to move around, while Hour is fixed to the Day Pillar, Month is Fixed to the Year pillar. : )