All the entries on the page state husbands surname and wife's maiden name apart from David Stevenson and Janet Stevenson.
The simple answer is that Stevenson was her maiden name as well as her married name. In Scotland, a woman does not legally change her surname on marriage, which is why you get the mother's maiden surname in many pre-1855 baptism records, and why all statutory records list the mother's maiden name as well as any married names.
The IGI at
https://familysearch.org/search/collection/igi lists four children of David Stevenson and Janet (maiden surname) Stevenson, all born in St Ninians.
Jane, 21 November 1856
Janet, 23 February 1859
Mary, 20 February 1860
Robert, 27 January 1862
If you have the marriage record, I assume that you are familiar with Scotland's People. If I were you I would look for the 1861 and 1871 census records for the family. I have not found them in the northern part of Scotland (which includes Stirlingshire) in the LDS' CD-ROM transcription, but someone else may have access to some other transcription that might help. This should narrow the search for Janet's (and David's) death certificates.
There is, in the above 1881 transcription, a widowed Janet Stevenson, aged 53 (so born 1827/8) in Muiralehouse, St Ninians, with unmarried sister Margaret
Stevenson, 43, and son Thomas, 26.