This score is a percentage number showing (Raced Laps + Led Laps - Total Laps) / Total Laps.
As an example, over Jeff Gordon's career, his LRL score is 6.5%
His distribution of LRLs over each race looks like this:
So his best races will be earn individual race LRLs above 90% and his worst races will be below 90%. the vast majority of races are between -20% and +20%. If you complete all 500 laps in a race but lead no laps, then you get a score of (500+0-500)/500 = 0. If you lead every single lap of a race, you get 100% score. If you complete 0 laps then you get a score of -100%.
So notice that in any given season, only the very best drivers will positive LRLs, as the vast majority of drivers end up in negative. Let's take a look at the last two years of top performers. This is a much better indicator of performance strength over the course of the year. Notice that the champion the last two years is the best LRL performer.
The 2008 Top 10 LRL rankings:
|7%||Dale Earnhardt, Jr.|
the 2007 Top 10 LRL rankings:
|1%||Martin Truex, Jr.|