A two story home can be off too. The stair case, open areas, etc - which can be difficult to measure.
I am not making excuses for the appraiser--it is very important to get things right. It appears that the difference in sqft did not cause a problem. If you have concerns contact the appraiser for more information. The appraiser's work file will have the sketch and measurements.
Also, read the report for comments on sqft.
I replaced my measuring tape years ago with a laser --sure helps in measuring those hard to reach spaces . The laser is connected to my iPad with an app that draws out the plan. I make sure everything is correct before leaving the property.
Was there a small conversion or addition that may have been done without permits? If so, that could be the discrepancy. If not, there may be other reasons. As other agents have suggested, you can check other sources such as the tax records. You can also check MLS history to see what it said about square footage. Sometimes homes are listed in MLS with square footage per seller instead of tax records, but that would show as well. If you want me to email you a copy of your tax records which will show what the county records state, simply send me a message and I will get it over to you.