Civilian GPS units usually have a very small factor of error , this is usually only a few meters in Lat, long and elevation.the one in pic is within this "few meter" range.My magellan does same thing,it will even give a different elevation for same peak on different days...not by much though!
I would trust the national geodetic survey's reading of 10,064'.If they can determine that Everest grows about 1/4" a year, I think they can tell within a few inches for Baldy.
Also, the Garmin website says that the elevation accuracy may be off by as much as +/- 400' depending on positions of satelites at time of reading.....wow!
Who knows for sure the true elevations? It is interesting, though, that the latest elevation for Mt. Whitney is 14,505 ft.--making it the only peak in the lower '48 over 14,500 ft. Almost more note-worthy is Mt. Williamson's new elevation listing--14,403 feet; a gain of over 28 feet! Do you suppose Mt. Barnard is over 14,000 feet again?