Having had time now to look at the earlier census, am not convinced that Mary and Martha are the same person, too big a difference in age, and on the 1871 Samuel 14 then Martha 8 makes me think Martha is daughter of Sarah, Mary probably left home to work. There's a Mary Smith aged 17 servant in Campton Beds. born Gt. Gransden, and in 1881 RG11/198 Folio 6 Page 5 a William & Mary Ann Marchmont, this Mary Ann also b. Little Gransden
Also, what about Samuel having married a different Martha from same village?
Barbara