Unfortunately many of the trees on Ancestry and similar websites have numerous errors, and these are copied by other people who do not check the original sources, and so the errors are perpetuated. There is only one way to be certain of your ancestry, and that is to start from yourself and check each previous generation one at a time with b/m/d certs, wills, M.I.s, electoral rolls, directories, other written sources, and oral history, then from 1901 add censuses and local history to the above sources, then further back add parish registers, etc.
Many people have wasted years following the wrong families because they started in the "middle" or used someone else's tree, and didn't follow this simple rule of genealogy.
The reason for searching all available sources is because you can't assume that a death cert. or MI, for example, is correct. It needs to be backed up with other evidence. Secondly, printed or published records (including indexes) cannot be assumed to be correct, but always need checking against the original sources.

Sue