Why can't the perpetual calendar be right?

Under normal circumstances, the main branch of the lunar month begins with the alternation of solar terms. For example, among the 24 solar terms, odd numbers are called festivals, even numbers are called qi, and the day when festivals begin is the day when the lunar month begins. The first month is based on beginning of spring, the second month is based on fright, and so on. If a year's beginning of spring falls on the sixth day of the first month, the first day of the first month to the fifth day of the first month is still counted as the dry expenditure of the previous year1February. The perpetual calendar you see may be designed for this purpose, so it will be different from the paper perpetual calendar.