They actually didn’t know why. They were surprised that it even happened. They were unsure whether Le Grand K lost mass or if the copies gained mass. The only thing they knew for sure was that the two were different. By definition, Le Grand K was a kilogram no matter what, so there was no way to prove it either way. Even if they could count the individual atoms when this occurred, they didn’t know how many individual atoms were originally in Le Grand K.
Ensuring that an important artifact is well-made doesn’t prove that they knew atoms were going to fall out of it. The original meter bar was made to be nice and was cut to specific dimensions and shapes even in directions it wasn’t intended to measure.
You seem to make condescending remarks about things which can be proven wrong with mere seconds on Google. Just like the thread about mount equipment, where you made a ridiculous and arrogant claim before I slapped you down with a Blizzard official support page showing you were wrong.
The irony here is absolutely staggering.
You do realize that if the pound is just some fraction of a kilogram, then a kilogram is a unit of weight, right? After all, the gram is just some fraction of a kilogram. All weights, not mass.
Also, the pound wasn’t always defined this way.
Again, no they didn’t and they never did.
Extrapolating is vastly different from measuring.
The unit of mass is the kilogram. The kilogram is the equivalent mass to a certain amount of energy given off by a photon. What part of balancing two things tells you how much energy photons are giving off? You’re extrapolating the mass based on weight and force.
A balance scale doesn’t even tell you the weight. It tells you a ratio. It measures how much force one thing applies compared to the force another thing applies. It will tell you this ratio regardless of whether or not you know anything about either object.
To clarify, take a balance scale. Pick a rock up off the ground and put it one side. Take a second rock and put it on the other side. The first rock hits the table and other rock gets lifted up. How much mass do they each have? You have no idea, because a balance scale doesn’t measure that.
What a balance scale does, assuming you have a precisely known control in a precisely known environment, is give you the variables you need to extrapolate the mass of the second thing.