Games : Is the leveling system in World of Warcraft designed assuming players typically complete only some of the quests?

I'm playing for the first time. I generally complete every quest before moving on to a new zone. I've found that as I neared completion of BC content on Outland, I'm close to level 80, while most of the sources online indicate that BC takes you from level 60-70. Am I an outlier or is this to be expected? Do people typically skip entire zones of content or complete only 60-70% of available quests?