Assay percentage

Why the product assay range is more than 100% what bases it is occurring.What type of cases the assay of particular product will increase more than 100%.
Plz give me clarity regarding this matter.

Laboratory error. Mostly +/- 2%

So why the manufacturer gives the produc range More than that value.
Ex:95% to 100%

i think, some dosage form like oily or suspension which can’t taken to person compactly due to some drug product sticky inside the primary package so they must make up the loos amount by increase the dose more 100% according their study