[Cruising Speed]

Local gravity may have changed slightly..............

Good thought, but the copies are being weighed in the small local gravity field as the original.

[Red Badger]

They can use mine: \

[BuffaloJack]

It also might be related to the moving of the magnetic north. If the poles are moving so is the earth’s liquid core, and local gravity has also changed.
It could also be dissolved gases in the metal.
Too many things could cause it. It will take a bunch of researchers several years to determine the cause.