Short-period giant planets tends to have relatively low albedos (i.e. low reflectivity) while long-period giant planets tend to have relatively high albedos (i.e. high reflectivity). Because HD 20782b spends the vast majority of its time far from its host star, it will have a high albedo similar to a long-period giant planet. HD 20782b spends only a very small amount of time around periastron and the planet’s high albedo atmosphere does not have enough time to react to the intense flux from its host star. As a consequence, HD 20782b, with its high albedo atmosphere, is able to reflect a larger amount of flux from its host star than a typical short-period giant planet. This makes HD 20782b easier to detect in reflected light when it is around periastron. Planets with highly eccentric orbits like HD 20782b can provide a good opportunity for the study of planetary atmosphere in reflected light.
Stephen R. Kane et al. (2015), “Evidence for Reflected Light from the Most Eccentric Exoplanet Known”, arXiv:1511.08679 [astro-ph.EP]