Yes this was very common. The registrar often lived a considerable distance away, and so people delayed registering the birth until they had to go into town on some other more important business, and then killed two birds with one stone. The births were often registered well after the 21 day deadline. So, as Conahy has said, people just moved the birth date to one that avoided a fine. I know two people, born in the 1930s, with “2 birthdays” as a consequence of the practice.