Complex sentence: The reason for this may be that the parent prefers to be called by his/her middle name instead of first, which is fairly common, or that the parent's middle name seems to fit the child's given name better.
Split into simpler sentences: The reason for this may be that the parent prefers to be called by the middle name instead of the first name; this is fairly common. It may also be that the parent's middle name seems to fit the child's given name better.