I would also like to add that trad (outdoor) climbing is really a different animal from indoor sport climbing, because indoor climbing is all face climbs and features on overhangs, and can't replicate real rock features such as cracks and chimneys or factors such as exposure, sustained length, and multiple pitches. (Although it's been a long time since I've been in a gym - maybe someone has made a hugely tall wall that requires multiple pitches!)
Someone who can face climb 5.10d might be stymied by a 5.9 crack, for example. And I remember passing a slow party on a high-elevation six-pitch 5.8 and suggesting that they bail, being told the leader was a 5.11 climber, and later finding out that they got stuck and ended up rapping down, leaving gear, and spending a miserable night in a gully as the weather came in. Turns out he was a 5.11 gym climber.
Finally, ratings are generally consistent in a single climbing area, but often widely vary from place to place. Older, historic areas may have been rated when 5.9 was the hardest; I remember climbing the "5.6" Durrance route on Devil's Tower in Wyoming, first ascent in 1938, and it was HARD. (I just did a quick google and see that it's now thought to be 5.7, with one 5.8 move...)
(Oh, and one correction to the comment above - the rating system used to be 5.0 to 5.9, not to 5.10. Because it was the decimal division of grade 5. Grade 6 was aid climbing - of course this has been replaced by the A system.)
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Someone who can face climb 5.10d might be stymied by a 5.9 crack, for example. And I remember passing a slow party on a high-elevation six-pitch 5.8 and suggesting that they bail, being told the leader was a 5.11 climber, and later finding out that they got stuck and ended up rapping down, leaving gear, and spending a miserable night in a gully as the weather came in. Turns out he was a 5.11 gym climber.
Finally, ratings are generally consistent in a single climbing area, but often widely vary from place to place. Older, historic areas may have been rated when 5.9 was the hardest; I remember climbing the "5.6" Durrance route on Devil's Tower in Wyoming, first ascent in 1938, and it was HARD. (I just did a quick google and see that it's now thought to be 5.7, with one 5.8 move...)
(Oh, and one correction to the comment above - the rating system used to be 5.0 to 5.9, not to 5.10. Because it was the decimal division of grade 5. Grade 6 was aid climbing - of course this has been replaced by the A system.)