Another explanation may be given.

Geoffrey wrote in an antiquarian age, when the ruins of Roman towns were being noted.

Both he and Henry of Huntingdon seem to have heard of the Silchester ruins, and both accordingly inserted the place into their pages.] [Footnote 3: The English mediaeval chronicles have sometimes been supposed to preserve facts otherwise forgotten about Roman times.

So far as I can judge, this is not the case, even with Henry of Huntingdon.
Henry, in the later editions of his work, borrowed a few facts from Geoffrey of Monmouth, which are wanting in his first edition (see the All Souls MS.; the truth is obscured in the Rolls Series text).

He also preserves one local tradition from Colchester: otherwise he contains nothing which need puzzle any inquirer.