Strength (18/xx): what does the number behind the slash mean?

The 2nd set of numbers behind str 18 indicates a percentile, the higher the better, until you get to 100 which in 2nd ADnD is reflected as 18/00 (100).

*Even though the number of 18/00 indicates 18 strength + 100%, it isn't equal to a strength of 19, it means you "almost" got 19.