Trulia imports their information from public sites. So that data came from a published source. Some Real Estate companies will not furnish any square footage info for fear of being sued if later discovered the information was incorrect. Some only give ranges, i.e. 2,500 - 3,500 sq ft. The problem with that is that sometimes the lower range is the data that is then imported into other websites and instead of showing the range, it shows the square footage as being 2,500.
In Tennessee, we can quote total square footage, but it must be noted as such. That is anything "under roof". However, if you don't preface it as such, then it is referred to as "heated square footage".
I am not saying this is your case, buy generally when the square footage is wrong, say with the tax assessor's data - the owner keeps his mouth shut so that he doesn't get charged more in taxes. When he is trying to sell his home, that is when the lower amount of square footage becomes a problem. Another red flag to consumers when there is a discrepancy in square footage is - was there an add on and was the proper permits pulled?????
Wish you well and best of luck,