Why is a meter a little too short?

The first prototype standard metre bar produced in 1875 was short by about 200 micrometres (0.02%) because polar flattening had been overestimated.

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In 1791, the metric unit called "the metre" was envisioned to be exactly one ten-millionth of the distance from the North Pole to the Equator. This is 1/4 of the Earth's total (polar) circumference which was determined (in 1790) to be 40,000,000 meters or 40,000 kilometers.

However, more refined measurement showed the Earth's polar circumference to be about 40,008 km, though still considerably shorter than the equatorial circumference of 40075 kilometers. This means that the semicircle used for the meter's definition is 2 meters longer than expected. However, rather than face adjusting the measured length of every object, the meter had been established in 1875 as the length marked on a platinum-iridium bar kept at 0 degrees C in the Bureau International des Poids et Mesures (Sevres, France).

*The definition was later amended to more precisely establish the length, based on an unchanging wavelength of light.