I am playing with srtm files and I dont quite understand my results. I am testing a lat lon of N39 W77 or 39.0,-77.0. I have tested using the python script from this post: Extracting elevation from .HGT file? and using gdallocationinfo. When I plot the point in google earth, it says the elevation should be 73-74m, gdal reports 81m. I'm curious why the difference?

gdallocationinfo N39W077.hgt -wgs84 -77 39
Report:
  Location: (0P,1200L)
  Band 1:
    Value: 81