Brita(Britha) Johannesdtr

I have some confusion about Brita(Britha) Johannesdtr. Possibly these are 2 different people. In the 1801 census, Britha is listed as 79 years old if I am looking at this right. I think she is the wife of Siur Johannesen. He also is 79. In the Bygdebok for Sogndal, Bruk l ca. 1750-1771 page 20. To me it appears that Brita(Britha) was born in 1712. This is a 10 year age difference from the 1801 census. In the Bygdebok, Siur is listed as being born in 1723. I tried to find Brita in the Archives to verify correct birth date but couldn't find her. Perhaps someone could let me know the correct birthdate for Brita. Thank you.