Like Guy mentioned, "Ancestry" fantasy trees are not a new phenomenon. Apart from wishful thinking there was also the need to prove lineage for personal gain, be it power, money or property. Also there were the "genealogists" such has Gustav Anjou who made up family trees, to order almost.
I suppose that William can be tracked back to Rollo but that was 9th? century, and Rollo's birth data is a mystery so far has I know.