It's actually quite easy to trace individuals in the mid-1800s in England. Even if they moved around quite a lot -- which my gggrandfather did -- it's easy to trace families through the 1841, 1851, 1861, 1871, 1881, 1891 and 1901 census returns. Granted, my ancestor had a fairly unique name, James Gilmore, so it's not like I'm looking up John Smith.
Once you find the person you're searching for, certain data do not change -- yes, I know, some census takers had atrocious handwriting, or it is written in another language, or the tenant was illiterate -- but all things considered there are trails our ancestors left behind.
For one thing, the birthplace generally remains the same. Sure, on one census the birthplace for James was Antrim and on another it was Ballyclare, but before you throw up your hands in despair you must realize that Ballyclare is a specific parish in Antrim. Next stop: find out how to obtain a 1929 birth certificate from Ballyclare parish.
Each census is a piece of the larger puzzle. Not all the information will be identical, but there are lots of little clues that lead us to pursue another line of investigation.