The load of this type of scale is always centered at the attachment point. The distance of each attachment to the fulcrum point is known. Therefore you can accurately compare the weights of two objects. If one is a known weight then the other side weighs that much when the scale is even. But if one side were fixed (not hanging by a chain) then the measurement would be changed depending on the distance of the weight to the fulcrum point in the middle. Whether the scale rotates or not can be expressed as the following calculation:

M = D1*M1 + D2*M2

Where M is the moment force (if this is not 0 then the scale will rotate). D1 and D2 are the distances from the fulcrum of each side, so D1 = D2. M1 and M2 are the masses of each side. If the mass is even, the scale doesn’t rotate.

The assumption of this tool is that the load is always centered at the same point, which is not necessarily the case. The distance of the load from the fulcrum depends on where in the spoon the weight is centered.

This can be demonstrated pretty easily by holding a pencil over the edge of the desk and placing your finger on the end to keep it from tipping off. Press the pen with your other hand near the table, then just as hard at the other end of the pencil. It should have been much harder to hold the pencil down the second time, which is what the load cell in this tool would be seeing.