Why are the contents of two calendars different?

Why are the contents of two calendars different? Because it was written by two people, there are errors in their own calculations, different calculation methods and different selection rules. I also read a document about heavenly stems and earthly branches, which recorded that I stared for one day less. I don't know why this day evaporated, as if everyone on the earth didn't wake up on this day, and I can't figure it out.