Could children born with no father noted on birth certificate just have unmarried parents as oppose to the mother not knowing who the father is?

Ancestors of mine were all registerred under the mother's maiden name with a diagonal line under 'father' in 1914, 1915 and 1920, their mother married in 1922 and all children took her husband's surname, is he likely to be the father too or just stepfather??
There was no father noted on any of the marriage certificates when the children got married either and after the mother died in 1931 it seems the father/stepfather disappeared...
Thought going off the years involved that WW1 might have something to do with it but I found his service record and he was only in army for a matter of months (Dec 1915 - June 1916) before being discharged due to deafness.
Any info on this would be much appreciated