Below is some explanation from Trulia on how they determine values. In it they admit there can be a 20% variance. When you changed your sq ft a whole different set of recent sales were likely looked at by their algorithm.
If you are looking for an accurate valuation of your home, let me know off line and I will be happy to send you a market value analysis in the next 24hrs!
If you are concerned about what value Trulia shows and want to try to correct it or "game" it, let me know, between us and Trulia, I think we can find a way to get it to be more accurate.
If you are thinking about getting ready to sell your place some time this year, you can learn a little more about me at these links
Then call me so we can discuss how to achieve your specific objectives.
"A Trulia Estimate is just that: an estimate of a home's worth. Our estimates use a ton of info, including recent sales of similar homes and home facts like number of bedrooms and bathrooms, square footage, and more to provide insight into what a home is worth. You can help us improve our accuracy by telling us what you think of your homeâ€™s Estimate, and by claiming your home and updating its facts... We measure accuracy by comparing (1) actual sales prices for recently sold homes over a 3 month period with (2) those homesâ€™ Estimates at the start of that 3-month period. For each county, we report the median error of these differences and the percentage of sales where Trulia Estimates were within 5%, 10% and 20% of the final sale prices...."
The above and more can be found at Trulia's website at this page http://www.trulia.com/trulia_estimates/