Easy, because it doesn't need correction on months ending on 30 or even 28 or 29.
I guess you mean a full calendar, with day, date and month indication. Some perpetual calendars (like Patek's) have a full calendar 'included', but just 'perpetual calendar' doesn't mean by default that it comes with all the day, date, month, year apertures
Same for annual calendar watches btw.
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