In this particular case, it would be harder to produce a fake signature from a non-existent person if the signatures in questions were digital and backed by a national identity number. The kind of that I'm submitting when signing documents at work.
I do not know how are they handled, but they probably get assigned one plausible date and it probably depends on the country.
In a sense, a birth date can be just as much an assigned number as an ID is. An ID can also have a checksum in it, potentially even a cryptographic one that only government can sign.