well, you seems to be on to something. These other kilos are kept at different places that have different gravitation (no mentioning that they control for that)

well, you seems to be on to something. These other kilos are kept at different places that have different gravitation (no mentioning that they control for that) - thus different time. So 1 year for IPK in Paris isn't a precise 1 year for its copy in another place. So whatever changes happen to the kilos they happen a bit quicker for some and a bit slower for the others.