That's definitely an artifact of Traveller's mainworld generation system, so it's less than ideal, but a way to think about it is to distinguish a surface ice sheet, like a glacier, from the world's bulk composition, which might be a mix of ice and rock at the surface. So for the hydrographics, it represents mostly pure ice chunks that you can toss into an onboard fuel refinery, where for icy crust, you'd probably need a mineral refinery to get to the water and then hydrogen without the rocky, dusty bits.