There could be a lot of reasons. For example, in my genealogy, the person who was listed as my 2nd great grandfather on my mother's maternal side, and in everyone's family tree, turned out not to be related to us. He did have a son with my 2nd great grandmother, and his decedents show up as DNA matches, however the decedents from his second marriage do not show up. This proves he wasn't my great grandmother's father. This also explains a group of DNA matches on my mother's side that were previously explainable. Using DNA triangulation from my mother and a half cousin descended from my great grandmother, and using the trees of the DNA matches, I have been able to identify who I think is my 2nd great grandfather.
So, to answer your question, it is possible that somewhere along your tree there is an ancestor listed that isn't actually your ancestor. Or there is a great aunt or uncle that isn't listed on your tree, perhaps was put up for adoption. There could be lots of explanations. If you want help trying to sort through it you can send me a message.