just look into the book. This is how it was defined. Notice, that an appropriate chalenge should use up 20% of the resources. If you read the whole paragraph, you would have noticed.
I have. And check out your own standards there, and apply them. A hill giant is the same CR as a level 7 barbarian. So 1 hill giant in an encounter is equivalent to 1 level 7 barbarian.
That's... utterly absurd. A hill giant does massive damage and has 102 hp, far more than a level 7 barbarian (assuming the level 7 barbarian rolls 6 12s for HP, he'd still need at 16 in con or so to equal that. With more reasonable rolls he'd need about a 26).
Yet the CR rules clearly state that 2 Hill Giants is just as challenging as 2 level 7 barbarians.
Nowadays we'd have the vernacular to properly express what is going on. The Barbarian is a standard level 7. The Hill Giant is an elite level 7.
Yet they both have the same CR. And that's why XP Budgets are much saner than CR - CR allows no differentiation between higher level monsters and more challenging monsters of a lower level, and that's not even getting into what it does to solos.