A scale works by compressing something and measuring how much that thing is compressed. In an analogue scale, that's usually a spring that turns a dial when compressed. In a digital scale, its usually a wire with a current being run through it - when the wire is compressed, the wire's resistance increases so the amount of current flowing decreases.

When you put a scale on a hard surface, there is nothing under the scale to compress, so 100% of the compression is focused on whatever inside the scale is measuring compression.

When you put a scale on a soft surface, like a carpet, the carpet functions as a continuation of the measuring surface inside of the scale. IE, some of the compressive force being applied to the scale is instead applied to the carpet, reducing the amount of force on the internal components of the scale.