This has been my experience as well. Doing ONLY normal/solo story, class, and side-quest content on a planet (so NOT including heroics or any group content, the bonus series, flashpoints, daily space combats, or PvP) will keep you right at appropriate level range. Doing ANY of that additional content quickly puts you ahead of the leveling curve.
I cannot fathom how someone can end up behind the leveling curve unless they are literally doing nothing but their class quests and skipping all other content.
If you are 3 levels below where you should be for Quesh... are you accidentally skipping the 32-36 planet (Taris for Empire, Balmorra for Republic)?
The attitudes here are a bit to harsh to elaborate.