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.
I purchased a home two years ago in Portland, OR (South East). At the time of my purchase, its price on Zillow was listed as ~$70K less than it was appraised for (I had two appraisals and both were within $1K of one another). Trulia listed the value within $1K of the two appraisals.
In two years time, the value on Zillow is listed as the original purchase price. On Trulia, the value is ~$60K more (it is based on an average appreciation of 8% annually of homes in my neighborhood).
I know a home's value is only what someone is willing to pay for it, but the disparity in estimates between those two services has always bothered me.