Your concern is well founded.....but the reality is that any time you engage computers to generate property value you remove the ability to consider many options including most importantly, the location. When a radius from the subject property is used to identify comparison property, they often overlook the often huge differences in sub-communities. Thus, you can be comparing an exclusive high end home with a moble home.
Using these resources can be frustrating but can also be a useful tool, if used by an individual that understands the differences within an area. Why the difference between Zillow and trulia? It's hard to say, but, truth be known, these resources should never be taken seriously.
There are two ways by which to arrive at an accurate appreciation for a property. First, by having is appraised by a licensed appraiser and secondly, having a real estate professional present you with a market comparison report.
Trulia's appraisal is $486,000.
Fanny Mae appraisal Nov/2012 was $1,130,000. Trulia is clearly the outlier here and not using property tax records, or anything else apparently.
Having never been inside your home, their Zestimates can't possibly be accurate. An algorithm will never take the place of a Realtor, and their local knowledge, giving you an accurate estimate of your home's market value.