That was my interpretation the other day - null yields 1900. An anomaly that needs to be explained nonetheless.
A lot of this seems hasty. This could be bad code in a bot that filled in the registration form online. Maybe the data that went into the DOB field somehow passed the front end error checking, and was sent to the DB only to be “rejected” and corrected. There are lots of reasons for a null anomaly that all point to poor/hasty coding and crap QA.