Have you ever wondered whether someone else’s research is truly accurate? Were they being thorough or just trying to tie in one or two lines? Might there have been an ulterior motive resulting in slanted research? For example, did a couple only have grandchildren from one of their nine children who lived to adulthood?
While this is possible it is not common and may require deeper research. I know I have discussed this before but I see more and more online trees simply copying bad research into their own trees and this is not a good practice.
I am now working on my third case of determining legal ownership of land which lacks a clear title. In order to do this, I must track down every living descendant of a daughter who died around 1899. The land has been passed down from generation to generation without a will or legal transfer. However, the only trees I can find relate to the line which stayed on or near the land. They might have little interest in finding other legal heirs to share whatever money they will get for selling the land.
Some other reasons why people may deliberately exclude a branch could be some family split such as a change in their religious denomination, criminal record, opposing side of hot issues like military draft, slave ownership, etc. The list could go on.
My point goes back to a disclaimer I have stated many times, “History IS what History WAS.” Whatever our ancestors did in the past should not be a reason to exclude them from our trees. If we are truly interested in researching our family history then we should be all-inclusive for the best accuracy and understanding.