The Chronicon Britannicum appears to indicate why there are differing dates for the Adventus suggested by the Historia Brittonum and the Historia Ecclesiastica. One of the possible dates indicated by the former is 428 and by the latter 447.
HB 66: “Vortigern reigned in Britain when Theodosius and Valentinian were consuls, and in the fourth year of his reign the Saxons came to Britain, in the consulship of Feliz and Taurus …”
HE II 14: “So King Edwin, with all the nobles of his race and a vast number of the common people, received the faith and regeneration by holy baptism in the eleventh year of his reign, that is in the year of our Lord 627 and about 180 years after the coming of the English to Britain.”
The CBrit inserts between the entries for the years 413 and 427 the following entry dated, out of sequence, to the year 447:
“Angli in majorem Britanniam venerunt, & Britones inde ejecerunt.”
This suggests that CCCCXLVII (447) may have been a corruption of CCCCXXVII (427), that is although the author was allocating the same date to the event as that in the HE, he was sequencing it in his list in line with a date close to that cited by the HB.