I came across a couple John and Elizabeth ROBERTSON
who acknowledged before the session? they had privately (married) on month of may
What does this mean compared to others on the page who
Lawfully proclaimed to marriage in the parish
It means that instead of having their banns proclaimed in the parish kirk and being married by the minister, they married by declaration before witnesses. This was a perfectly valid marriage, though such marriages were, not surprisingly, frowned on by the kirk.
Does the fact that they have same surname reflect their married status
No.
or could her maiden surname be ROBERTSON. too ?
Yes. Under Scots law a woman does not lose her maiden surname on marriage. This is why you get mothers' maiden surnames in baptism records, and you also find married women listed in the census under their own names, not their husbands' names.