dunderm, I
think you're basically asking two different questions here, so I'll answer them as such.
1) Why is offence (BAB) better than defence (Dodge/Parry), i.e. why is it so easy for the twin to hit his brother?
Well, this is just the way d20 combat is set up; when you increase in level it will get easier and easier to hit an equal opponent. Basically, at high levels, your primary attack will almost always hit, while your secondary, tertiary and quaternary attacks perhaps won't be as lucky.
What balances against this is off course that you get more and more hit points as you level up. So, with increasing levels, you'll get hit more often but you can also soak up those hits. This is something that many people find very unrealistic - heck, I used to be one of them until I realised how well d20 combat works in play!
As I see it, the trick is to regard hit points as fatigue/combat capacity and not actual flesh wounds.
2) Why are different classes better at defence relative to their offence, i.e. why are different twin pairs better at hitting each other than others?
What you seem to be advocating is that defence should be completely correlated to offence, for example that defence should always be equal to BAB/2 or something (as for the Nomad and Borderer).
While that could absolutely have been one way to go (you would basically just have one "overall combat score"), Mongoose have for the Conan game chosen to add a little more variety so you get classes that are good offensively/bad defensively, good at both etc. etc. Furthermore, they've taken it one step further and divided defence into two; Dodge and Parry.
I dunno, I think I like it the way it is; gives rise to different sorts of tactics methinks. Haven't really thought about it all that hard, though.
).
