I think simply it was because that was how many logical pieces it fell into when you broke it down. Are you thinking there is more significance to the number?
Only that it's a direct reference to the number of days in the week.
Here's God's Work Report
Day One, created heavens, Earth and separated light from darkness.
Day Two, made vault to separate sky from water.
Day Three, gathered water in one place and dry land in another, created vegetation.
Day Four, created stars, sun and moon.
Day Five, created fish and birds.
Day Six, created land animals, man and appointed man the boss.
Day Seven, Rest
You say you can logically break these jobs down into seven periods I very much doubt you can.
Scientifically speaking the days are out of sequence. I read that workaround you came up with to explain why the stars, sun and moon appeared after the earth, dry land, seas and vegetation but that's not what the Bible says...