Philologists are fond of genealogical charts detailing the precise scheme of inheritance between various manuscripts of the same work. Chronological accuracy is sought after, and the influencers must not be confused with the influenced.
This simplified map is not meant to be a proper stemma of Arthurian literature,but is rather meant to say “this makes sense as the before of that.” It is meant to help the modern reader, but without an attempt at making strict chronological sense of the works.
The reasons are many: not all medieval texts can be precisely dated; the recopying process of an early text at a later date often introduced contamination from more recent development, so that when the original is lost, it is hard to figure out what came first; and finally, the reader intended here is not working on a thesis of the manuscript tradition.
For instance, it is not altogether clear whether Ulrich von Zatzikhoven Lanzelet was composed before or after Chrétien’s Lancelot, but there is a suspicion that he was working from a more ancient version of the story, which did not contain the adulterous relationship with Queen Guinevere. So it would make sense for the reader working “chronologically” to read Lanzelet before Chrétien’s Lancelot, and the prose Lancelot in order to appreciate the expansion of the tradition.