Not to diminish your achievement of doing a good job at healing for the first time taking your shaman through naxx (the world needs more shaman), but your lack of overhealing can be attributed to your gear. Its not the best gear you could get I would imagine, as such your spell power is lower than the other people.

If a tank is missing 1000 health, and you heal it for 900, yes the tank will live and you won't have any overhealing, if the same tank gets healed by someone with much better gear and gets healed for 1200, then ya, they will get a big overheal. Overhealing is only an issue if people start running out of mana. The new changes to wrath where all us healers only use our top rank spells and never downrank make overhealing a non issue when it comes to looking at meters.

Resto Shamans lend themselves to low overhealing in the hands of a good player. Thanks to Tidal Waves you can throw out single target heals extremely quickly without much worry that another healer will get there first.

In addition, Earth Shield, Chain Heal and Ancestral Awakening are "smart heals". The former will only heal when the target takes damage, and the latter two will specifically target the people who need it most. The only real potential for overhealing is the primary target for Chain Heal, or a poorly targeted/timed Healing Wave.

Meanwhile, Priests and Paladins will generally have a lot of overheal. Many of their healing spells will hit additional targets whether they need the healing or not, and Holy Priests specifically have a talent which refunds mana on overheals. I can't think of a reason for well played Resto Druid to have a lot of overheal.

So essentially, if you play your Shaman well it's not particularly surprising to find you have low overheal. It essentially boils down to avoiding heal sniping and carefully choosing Chain Heal targets. It sounds like you're well on your way there.