That may very well be true! If there are some 'cross-overs' (aka generation collapse), he might actually be a grandfather for both ... but not necessarily.
There are often, of course, many different "paths" to a particular ancestor's profile. Real families are not often nice, neat binary branching trees. More like tangled bushes.
You may want to use the HistorySearch tool to check for "problem" profiles in the paths; they can cause the relationship (path finding) algorithm to get confused.
https://historylinktools.herokuapp.com/history ... Geni Project here: https://www.geni.com/projects/HistoryLink/14021
'cousin' relationships are typically "connected" through a family sibling (or aunt or uncle, etc). Being listed as a 'cousin X removed' typically is NOT an indication of being a direct descendant. Look at the expanded details in the Relationship panel to see how the actual connection path might "meander" through siblings and perhaps across marriages.
re; "If you are the descedant of a person you are normally the same to his/her spouse (if they only have one)."
That is true, ... however ... Geni may find a different path that is shorter, depending on the siblings of that spouse (as well as a variety of technical factors having to do with "caching" of path segments for efficiency).
See more at this project:
https://www.geni.com/projects/Working-with-Relationships/17570