If you're sneaking past mobs instead of fighting them, that could be a very large part of why you are finding yourself underleveled. At my current level (30) I find killing a single mob at my level gives me around 250 xp (just a normal mob, mind. the ranked/boss mobs give more usually), and the quest reward might only give me a paltry 2000. If I kill something like 40 mobs on the quest though, that 2000 xp suddenly becomes 12000 xp, which is something like 25% of a level.
For those that have answered "Yes" - have they found all the sky shards and achieved the quest accomplishment for each area they have finished (even using a wiki to finish the sky shards if necessary as at least that will show areas they may have missed)
In my experience it is possible to just follow the main quest and blast through the area which is great for PVP'ers, or group players who want to get their experience in other ways but may cause this problem for other players as there are so many extra quests and bosses etc that only appear when you've read the piece of paper lying on the ground or spoken to a random NPC.
Personally I love this system as I read and talk to every thing - but it may be a new approach to some players leading them to believe that the extra quests are not ingame.
Just so you do know, Skyshards do not grant experience at least not for me. If you are getting XP from the them then I dare say I'm further bugged then I thought.
Are you saying there are completely hidden quests on completely random NPCs that are not even of interest? Do they have the quest mark on them at all? Because if not that is a *** terrible design choice on the devs part. Not everyone enjoys talking to every single NPC. Every NPC I talk to, I talk to because they are at least of some interest to me whether that interest is simply because they are a vendor or they are a quest giver. I have played every TES game and I always play the same way, quests should feel natural not like Where's Wally.