Hi Beth, I also think you need to look at the original of the 1851 census. There are quite a few errors in the transcriptions.
Ann won't have a birth certificate will she, if her age on the census is correct, she was born about 1835.
I have checked the IGI and there is an Ann Downing born 25 December 1834, Christened 26 January 1835, St Martin's Birmingham. Father John Downing mother Mary.
I can't find a marriage for John and Mary, but there is a marriage for Mary Downing and Edward Calloway 20 April 1845 St Thomas Birmingham.
so it would seem that Mary married twice and Ann is the daughter of her frist marriage.