Hello, from what they say on the wikipedia page : the variation in mass is due mainly by 'contamination' , that is, manipulations, cleaning, transport (from one room to the other), etc. They explain also that it could be caused by adsorption : https://en.wikipedia.org/wiki/Adsorption a chimical phenomena that occurs on the surface of an object. They were supposed to measure the prototypes every 50 years, minimizing the manipulation, the 'grave' which is the prototype 1 is named IPK and the copies are named P20, P21, P40, and were distributed around the world for security. They realized that they where varying in mass over time, shifting slowly. This is why they decided to make the kilogram definition based on the Planck constant , there was also a project to define the kilogram using a specific portion of 1 element based on the mole : 6.022 x 10^23 atoms, they made 1 prototype of a <perfect> sphere for it but that idea was abandoned. Personaly, I prefered the second definition (the sphere), mainly because it uses Pi. ;-). Best regards, Simon Plouffe