This is a perfect examply of why it is important to employ the expertise of a real estate professional.
Trulia's property value estimates are imperfect. For example, they will often compare properties in Winchell with properties in Oakwood. While they are both great neighborhoods, values are quite disparate for a number of reasons. The process also does not differentiate between REO properties and fair market value properties. REO )bank owned) properties tend to sell for a lower price because, amon other reasons, they usually have deferred maintenance and there is not disclosure.
The other issue likely affecting the value estimate is that, again for multiple resasons, there has been a decline in REO properties sold in the area. The value you are seeing is likely more reflective of fair market value rather than distressed value.
I realize this answer is largely conjecture given that I don't have an address or the list of comparables used in either estimate. Hopefully this has still given you insight into the volatility you have noticed. Feel free to call me at 808-0324 if I might be of more assistance.