The FSA does not use GPS. They trace aerial photographs. You can do this yourself on the website for NRCS soil maps. As others have mentioned, a hilly site will show less area from an aerial photo than will an accurate GPS measurement.
This isn't a huge effect -- for a field with a steady 10% grade the aerial photo method will give 99.5 percent of the true area measured along the slope of the hill.
Non-differential GPS, even with WAAS, is not accurate enough to measure field area or distance of travel with 0.5 percent of precision. You need a separate, local transmitter to begin to approach that accuracy, AND the operator has to set up the equipment and use it correctly.
One example way to mis-use the equipment would be to use the GPS to measure distance traveled, then multiply by swath width to come up with "area". Naturally, this will over-estimate area by the amount that any swaths overlap each other.
I would query the operator closely as to exactly HOW the gps calculates area, when the equipment was last calibrated, and what comparisons he's made against between reported area surveyed area. If the answer is "I push the button and it give me the area, I don't know how it does it, I've never tested it", then I'd be unlikely to accept his numbers unless the difference was too small to argue about.
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