@Rivz You could be getting lucky with drops and not getting extra EXP from the amount you're supposed to kill to complete a quest. The guide gives you EXP thresholds to grind to if you're not already at that threshold. If, even after following the EXP grind steps, you're still behind, then grind some more whenever you're supposed to be at a certain level. Like after a quest complete or quest turn in, you're supposed to ding and you're not dinged, then when you're currently questing or next out questing again, you could kill every mob you pass in between quests (instead of standing in one spot to grind). Do this until you're caught up, and if you ever fall behind again.
Me, I've been consistently at least a level and a half ahead of the guide since either the pre-10s or not long after level 10. At level 24 with 31% EXP, I've just started taking quests in Menethil Harbor, to start questing there for the first time, and guide thinks I should be level 23.
And I haven't been grinding as I travel.
RNG I guess.