I'm only speculating and commenting now, because it's an interesting question and I enjoy discussing and learning more about family histories. So here goes:
I guess it could be partly because surnames were not an established thing in those days,yet titles and birth locations or family origins certainly were. Also popular nicknames, and anything that could serve to indicate who sired them. An unusual nickname for a famous patriarch could serve the purpose, in many cases.
The assumption by the lower-ranking younger progeny of a particular form of the patriarch's nickname was possibly used by some clerks and historians simply as a means to identify their origins or blood lines, while the heir apparent had his own name and title that was enough to clearly identify him as the son of the man from whom he inherited the title. And apparently the eldest son and heir additionally gained his wife's inherited title as 3rd earl of Chester.
The second son acquired a title somehow, and was distinguished with the honor of carrying a classier form of the nickname ("the" Meschin), while the younger siblings were called "of" Meschin.
On a side note, just an idle observation and personal opinion: The repetition of "de Bayeux" in Ranulph II's display name seems redundant although that may be the correct form, for all I know. I would think it could be tacitly assumed that he was "from Bayeux" by virtue of the fact that he inherited the title as lord of that region.