The reason is because the wind in Portsmouth was coming straight off the sea (SSE) but on the IOW only the South, Southwest and right on beach on the East coast had an onshore wind, so away from those areas the wind had traveled over at least 5 miles of land and was therefor much colder, so places such as Ryde, Wootton, Brading, Cowes, even Newport saw snow but Niton, Ventnor (near sea level) Compton, Freshwater all saw only rain with a few blobs of sleet. Even Sandown only saw sleety bits whilst a mile up the road in Shanklin had snow.