I agree with Sinann regarding the listing by ancestry of a match as a 'half' brother/cousin or 4th cousin once removed, for example.
I have a branch where the Surname has been given as :- McTaig/McTague/McTaigue/Montague. Common ancestors where the first common ancestors are a marriage/couple where one of them is from this family, are often given as half relatives. In these situations the family trees of the 2 matches will have one partner with identical naming, while the other partner has the surname in two of the alternative forms.
Similarly, there are times when you wonder why someone isn't a common ancestor. Again with the family group mentioned above, in my tree there is an example where I didn't have the name of the partner (they emigrated and married in the US, which I am unaware of), so if ancestry found the connection, it would be listed as a 'half' relative. But this is not the case, because the 2 trees have the Mctaig partner, spelled in alternate ways. and so the match is not listed as a common ancestor at all!
This sort of problem is quite common, especially if the family were illiterate (which was for a long time the norm), and wouldnt know the 'right' or wrong spelling for their name, and would not have been able to indicate it was wrong when written on a document that a century later becomes the source of names in all our family trees!
Surnames are not likely to be the only source of this problem. I'm sure the same thing will happen with First names that can be written in many alternative versions!
Jane