You say each attribute would start at 2. My suggestion is that if a character has their favoured attribute be one that is already a part of their codex, they start with 4 or 5 in it. This may or may not allow the attribute to go above 10. It would give a reason that someone might make a character who has a favoured attribute be one of the ones their codex excels at. Otherwise even if the fluff would point to that, there is no mechanical reason for them to do so (it just means they have less attributes that cost less xp to upgrade.

Owrtho