Did she lie about her age?

Working on the 1861 census I had trouble with the entry for Ann Goatcher of 19 Vine Street. In 1861 she was 30 and living with her son William (10) and daughter Betty (12). The birthplace was unclear, but looked like 'Heartham'.
Going back ten years to the 1851 census, I was fairly sure I found the same Ann Goatcher living in Eastergate with husband George, and Ann (7), Jane (5) and Elizabeth (2). The birthplace was given as Eartham which would fit with Heartham in 1861, but her age was shown as 31. Could it be the same person, and did she lie about her ages different?
There were several other Ann Goatchers in the 1851 census, however, none seemed likely: One was 60 years old. Another was 32, but was married to William with no sign of daughter Betty (or Elizabeth).  A third was aged 30, but was married to Richard, living in Washington, and her youngest daughter was Ellen.
Returning to the 1861 census there are 4 other Ann Goatchers:
One aged 70 living in Eastergate with husband George, one aged 65 living as a housekeeper in Pulborough, one aged 45 living in Storrington with husband William, and one aged 39 living with husband Richard in Washington.
None of these seemed to shed any light, so I concluded that the Ann Goatcher in Vine Street, 1861 was probably the one who had lived in Eastergate in 1851. That would mean that she was born around 1821 in Eartham in Sussex and married George, probably from the Eastergate family.
After a couple of hours, I was beginning to feel that I knew her, but if she is the same person, why did she lie about her age? Was it vanity?