Why an earlier birth date for Catherine seems more likely

Why suggest that Catherine was born earlier than June 1264?

The next pregnancy and the customary period of rest after childbirth favour an earlier date, but do not establish when she was born.

The explanation refers to a customary period of rest and seclusion after childbirth, especially for royal women. Placing that interval alongside Joan's following birth makes a very late date for Catherine's birth harder to reconcile. The argument favours an earlier date while leaving the year unresolved.