Disclaimer: this is what I think, it may not be 100% accurate The first one is the characters base THAC0 derived from his class and his current level i.e. a level 1 fighter has 20 THAC0, a level 2 fighter has 19 THAC0 etc. The second number is his current actual THAC0 taking into account the bonuses for >18 strength, weapon proficiency and the to-hit bonus from magic weapons etc.
Blackrose, you're right, the number in parenthesis is the THAC0 "after modifiers".
T'was the same in Planescape IIRC...In BG2 it's more clear, there are 2 lines "base THAC0" and "Adjusted THAC0".
There are 2 THACO values for each char ? 
The first one is the characters base THAC0 derived from his class and his current level i.e. a level 1 fighter has 20 THAC0, a level 2 fighter has 19 THAC0 etc. The second number is his current actual THAC0 taking into account the bonuses for >18 strength, weapon proficiency and the to-hit bonus from magic weapons etc.
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