It's hard to say for sure. Every book I've read just says that it's that way. I suspect, however, that it has to do with the position of the moon relative to the planet at any particular hour.

“Location Location Location” you heard that one before there’s the connection
You make some very advanced questions in your query some of which are ancient in nature.
Time is sectioned into angelic watches as well but for a example of your questions complexity which I’m sure you didn’t intend it brings up these watches but for practical application. I will say look at the day sunrise to sunset it isn’t always a 12 hour cycle lets say the hours become longer or shorter depending on that alone for example 5am sunrise to 5 pm sunst would work out a 12 hour day but 7 am to 5pm would make the hours 2 hours less so your hours are only 50 minutes long instead of 60 minutes. Also the stars and moon have moved in there (the L word ) Locations these 5,768 years thereby complicating the correct insight into the basic core of the whole of it. Or as they say close but no cigar. So its a lot more complicated. Hmm where’s a astronomer. Good question you asked and it reminds me of a quote
" Great People talk about Ideas....
Average People talk about Events
Small People Talk about People..."
Oh and for Alaska, Antarctica there’s more questions and answers weeeeeeeeeeeee
Bryan