Most of the difference is likely just a matter of data. If Trulia has better data than Zillow, their estimate will likely be closer to reality. There is the added difficulty of coming up with what's basically one methodology that has to work everywhere. My guess is that if you're in a funky place, like on the border of two neighborhoods, or if you're in a rural area, your estimates are likely to be further from reality than if you live in a cookie-cutter subdivision or in a downtown area.

The point is: yes, it's frustrating, but it's also complicated.