Attack Rating...how is it calculated?

[Seyal]

I've noticed a connection to multiple attacks and
attack rating. It seems when the attack rating is 27
or more for a weapon, a character will get 2 attacks
on that weapon. I noticed this after equipping my rogue
with a thief sheild I got in a battle instead of a dagger
in her off-hand, and saw her dagger attack rating go from
26 to 29 and her attacks from 1/1 to 2/1 for her primary
weapon (a dagger).

As an experiment I took the wakizaki from my samurai,
and her attack rating went from 26 to 28 and her attacks
from 1/1 to 2/2 with her Bloodlust sword (I believe that's
what its called. I haven't checked with her enchanted
katana yet)

My ranger gets 2 attacks with her bow and her attack rating
is 28 with her bow.

So I wonder how attack rating is calculated and is this
all it does (determine multiple attacks)? Also, why does
it decrease with 2 weapons? I can understand the off-hand
weapon's rating being lower but why does the primary weapon
decrease when the character has skill with dual weapons and
with both of the wielded weapons?