Hi Brad! The discrepancy in the birth years is not really that much, considering! I have seen census returns over the years where people only age 6 years in the 10 years between enumerations! And they live long enough to be in 4 to 6 enumerations as an adult!
If you can get any sense of who is giving the info to the census taker, you might know what to make of the different birthplaces. For instance, by the time she is elderly, if she is living with relatives from one side of the family, they might know more about their side than the other side. For the 1850 census, she is still quite young. If she is unmarried and not living at home, it might be an employer giving the info, and they might have a vague idea that she comes from Up North Somewhere. If her current residence aligns with the info about her birth, it might just be that the informant didn't really know and figured she must be from Around Here Somewhere.
Remember that neither the informant nor the enumerator had any idea we were going to be poring over these records trying to see into the past!
Good luck with it!
Cheers
Shirlea
PS - could you put some info and maybe links about those censuses in her profile?