  We consider a class of unstable surface growth models, z_t = -\partial_x J,
developing a mound structure of size lambda and displaying a perpetual
coarsening process, i.e. an endless increase in time of lambda. The coarsening
exponents n, defined by the growth law of the mound size lambda with time,
lambda=t^n, were previously found by numerical integration of the growth
equations [A. Torcini and P. Politi, Eur. Phys. J. B 25, 519 (2002)]. Recent
analytical work now allows to interpret such findings as finite time effective
exponents. The asymptotic exponents are shown to appear at so large time that
cannot be reached by direct integration of the growth equations. The reason for
the appearance of effective exponents is clearly identified.
