Recent apparent discrepancies between results from low-energy electron and X-ray diffraction concerning a reduced in-plane lattice parameter of Cu(100) are resolved in favour of an uncontracted surface. We show that neglecting the energy dependence of the inner potential in the electron intensity analysis simulates reduced structural parameters when a precision level of about 0.01Å is reached. As today this level is frequently claimed, our finding is of general relevance, in particular when the in-plane lattice parameter is not precisely known, as in epitaxial growth, for example.