It's exceptional Strength, available only to warriors
Baldurs Gate Enhanced Edition does indeed use the AD&D 2e rules as the chief designer confirmed.
In AD&D 2e, members of the Warrior group of classes (including fighters, rangers and paladins), with 18 Strength have what is called exceptional Strength.
This means that members of other classes can only have 18 Strength, whereas warriors can have have 18/01 to 18/00 Strength, giving warriors a big bonus in combat.
In the pen-and-paper game, if a Warrior has 18 Strength, the player rolls percentile dice, and records this after a slash after the 18, hence 18/57 etc.
The benefits can be seen in the Strength Table (AD&D 2e Player's Handbook, p. 14) duplicated on this Baldur's Gate wiki.
How does it work?
Exceptional Strength is divided into the following tiers:
Thus each tier gets more and more unlikely (and powerful), and 18/00 is truly remarkable.
For example a non-warrior with 18 Strength has +1 Hit Probability, +2 Damage on attacks, whereas a warrior with 18/01 (the lowest) has +1/+3, and a warrior with 18/00 (the highest) gets +3 to hit and a whopping +6 to damage. This is a huge combat advantage in a system where 15 Strength gives you no bonuses to hit or damage.
Note: In Baldur's Gate it is possible for a starting half-orc character to have 19 Strength, which gives +3 to hit, +7 to damage. In pen-and-paper AD&D 2e it is extremely rare for a player character to have Strength over 18, hence this subdivision for warriors. Although the Strength table went up to 25, these scores were usually reserved for powerful monsters, gods, the effects of powerful spells or magic items etc.
See also: Where did exceptional ("percentile") strength come from?