Different records on your ancestor will provide different information. Sometimes the differences are slight, like an age off by a few years. Usually there is other information on the record that leads the researcher to conclude the record is on the same person–same name, right location, other details on the document match.
Remember that it is uncommon for every record on your ancestor to be entirely consistent. Usually different records should be fairly consistent. Try and explain the reasons for inconsistencies, if you know them, in your research notes. If two records appear to be the same person and the inconsistencies are great, you may wish to consider whether the two records are even for the same person.
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I understand small differences. Especially on death certificates when the informant does not know the information required. Sometimes the differences are overwhelming. I have a great grandmother whose birth is different in every piece of paper: documents, census reports, family trees,… I don't think she wanted anyone to know how old she was.
Colleen
http://www.pasqualefamily.net/web/