It does seem odd if they were witnesses as there is a separate column in Scottish Marriage registers for the witnesses.
What is in the witness column on this certificate?
I've just checked through many pages of my saved images for deviations to the statutory wedding format.
Column 6 is for the witnesses to the marriage. For those who have made their mark, the usual form is name with a cross between first name and surname and the signatures of witnesses to the mark beneath.
It would seem that, as Isobel has said, the (mark) has been omitted in this case.
Gadget