There is a DANIEL McMULLAN Birth in 1899, Glasgow City, Milton, on Scotland's People.
And there is a similar MARGARET McMULLAN Birth in the same area etc, 1904.
There is a WILLIAM MARSHALL to MARGARET McMULLAN [Surname variants] Marriage in 1922, Glaasgow City, Hillhead.
Also Death ofMargaret McMullan born in 1904, died 1979.
Whilst I have not specifically the names of Grandparents in place of Parents on a Marriage Record, I have found the names of a couple given as "Parents" who would have been aged definitely under 10 when the Bride was born!
There was some sort of connection, as the male Surname of the couple was the same as the Btide and therefore the same as her correct father when I found the Bride's Birth Record.
It may bethat the couple were informal guardians etc of the Bride - but nothing noted in the Marriage Record.
And the couple had married only four years earlier. In fact this was the clue to there being discrepancies.