I see the name as Christen - no second 'i'....
True the person is named as this on the OPR record, but I believe it just to be a normal variation of the same name as was common at this time. The name was written as the Clerk thought it to be spelled. This could vary.
In the 1841 and 1851 census she is named as Christian and also in the 1851 newspaper article commenting on her death and long life. I still feel confident that Christen and Chistian are just variations.
EDIT:
There also appears to be a connection between the surname Campbell and the Bruces of this area. On the above OPR baptism record a witness is a
James Campbell.
My 4x great-grandmother is Christian Bruce. She marries a William Anderson, about 1790. On two of their children's baptism OPR entries another Campbell of Aberdour appears. They relate to;
John Anderson b.1793 Haddoleys, Gamrie. Witnesses: Alexander Walker of Logie and
George Campbell of Aberdour.
Robert Anderson b.1802 Haddoleys, Gamrie. Witnesses:
George Campbell of Aberdour and Alexander Foulie of Glens of Troup.
Robert is the last child I have for William Anderson and Christian Bruce.
My only slight problem with the above is that Christ(i)an is having children slightly later than normal. She appears to marry late as well. Though, I do not know if this is her second marriage or not. The first child of the marriage appears to be a Franc(i/e)s Anderson 1791. So mid 30s for Chrstian.
If the OPR baptism is of her she'd be mid 40s having her last child Robert. Having said that, the 1851 census she is stated as being 97 years old, this also concurrs with the newspaper article in December 1851 which mentions her death. In reality she would've been about 95, so 2 years out.
I'm still leaning towards the theory that the 1756 OPR is of her.