Hello Josh and thanks for your post.
It is not surprising that your measurements, which I presume were made by measuring each room and then adding the square footages together, was about 200 feet short of the stated amount. The tax assessor will typically use the square footage stated by the home builder or developer. While the plans may certainly call for a home of 1300 square feet, often the "as built" (as we call it in the industry) is not the same as the filed plans, so that anywhere from 20-100 feet of home can be missing. Also, people typically forget that the flooring on which "walls" are placed are also considered in the calculation, so it is not at all uncommon that walls within the home take up as much as or more than 200-300 square feet of space. Consider that the average wall is 6 inches thick, so that every two linear feet of wall equals one square foot of floor space.
As Tom and John noted below, your best and most accurate calculation of square footage will come from a qualified and knowledgeable appraiser who will measure the exterior of your home and deduct from the calculations those areas that are not heated or habitable.
Good luck and happy home ownership!
Area Pro Realty
San Jose, CA