As my disclaimer
says, the climbing ratings are very subjective. Routes are rated by
those who put them up and then "general consensus" of those
who follow it will confirm the rating. Your best bet is to ask local
climbers their opinion of the ratings you find in a guide book. Personally,
I find climbs in Yosemite to be slightly harder than climbs in Joshua
Tree. Remember that not too long ago, the highest rating a climb would
be given was a 5.10! So climbing areas developed a long time ago may
have some steep 10s that might equate to 11s in the newer areas.
With that in mind,
here's my short explanation of how routes are given certain ratings.
Rating are based on: how easy/hard the moves on the route are, hold
availability and quality, and finally, endurance and exposure factors.
Climbs are generally rated by their hardest "crux" move
but if a route is consistently hard throughout the climb it might
be given a higher rating based on the "endurance" needed
to complete it.
Feel free to comment
on this article and/or my "novice - crazy rock diety" breakdown
in the chart above on the forum