Okay, okay.
Imagine you were floating in the air above the scale, what would the scale read? It would read 0kg. Since weight is affected by gravity.
Same thing goes for the flies, they aren't contributing any weight to it if they're flying. The picture shows that the flies are indeed flying in the jar. The scale will read the weight of the jar, or 1kg give or take. The scale will only read something higher if the flies drop down and is pulled by gravity.
tl:dr version: Learn the difference between mass and weights damn it! This shit is like physics 101.
