In the past century and half, the height measurement has changed as more accurate survey techniques became available.
When the Great Trigonometric Survey Of India was done in 1856, surveyors triangulating from the plains found the height to be exactly 29,000ft. However, Sir George Everest, who headed the Survey, thought people would think they rounded off the figure, and added 2ft to it just so it sounded more accurate.
The true height differ depending on where it is measured from – mean sea level, or the centre of the Earth. Because of the Earth’s equatorial bulge, Andean peaks can actually be considered ‘higher’ than Mt Everest.
Measurements have also differed in the past because some take the elevation of the rock at the summit, while others also add the icing on top. The snow cornices on the peak are about 4m thick, depending on the season, wind patterns, and the impact of global warming.
Global heating melts mountains, Nepali Times