There's no such thing as an exact value in engineering. EVERYTHING has a tolerance.
- That hole you drilled? Only accurate to within ±1mm.
- The elastic modulus of steel? It's a statistical range that depends on the microcrystalline arrangement of the atoms.
- The shear stress of your wing spars? Depends on the cross-section which is not accurate because of tolerances in the manufacturing process.
- Horsepower you get from fuel? Depends on chemical composition, which is again subject to tolerances and impurities introduced when it is refined and processed.