You have created a Smart List with a filter for a date type field and you are using the constraints either “before” or “on or before” a particular date. However, records with empty value on those field get qualified.
When you create a Smart List using a date field filter (e.g., Date of Birth, SFDC Created Date) and use the “before” or “on or before” constraints, the Smart List will also include people who have no value in said date field. This is an expected behavior as by default Null value is treated as a timestamp with a default value (e.g Jan 1 1900). This is similar to a token having default value and this is also mentioned as a caution within our knowledge base document over here.
The work around is to use an addition filter or to make sure that the date value is populated for all the records. You can learn the importance of date stamping in this Champion blog post.