There are several reasons why someone might not have the last name you think that they do or why they may be listed in a record with a last name of which you are unaware.
A female relative could have had one more husband than you think she did—resulting in a reference to her with a name you don’t know.
A child may be listed under the name of an unknown step-father. Without a last name it can be difficult to find someone.
The man you think is someone’s father may actually have been the step-father, adopted father, etc. This again can cause the same person to be listed in different records with different names.
And you just may simply have the last name wrong.