Allah knows best about it. I have a similar one on a poster that I bought. Personally I would say that the further back in time it goes the more likely it is that there are large gaps between the famous names where there were many many individuals who were not named. I would use the analogy of this. It is like we may say Jesus (as) was a 'son of David' or Muhammad (saws) was a 'son of Ishmael' and it is true in one sense, but if taken in a genealogically literal way then it misses out the many generations that were between them. However the later in time the genealogy goes the better it is remembered and therefore the more accurate in the sense of naming everyone. Does this make sense to you?