I'm following the Horde side right now, and i always come up a few levels short for each section. When i was supposed to be 25 at the end of Silverpine, i was 22. When i was supposed to be 45 at the end of Hillsbrad, i wasn't even 40.
Maybe it's entirely down to how many rares you meet along the way? I still feel like this is a fast route, but i'm obviously missing something.