So you see, the freeze is driven, primarily, by local process: the air "here" gets cold (without the sun), and it freezes sea surface "here" to ice; but the melt is primarily driven by inter-regional processes: the ice "here" is cold and it's reflecting most of sunlight, not allowing water below to warm up and only slowly warming up itself, - but water and winds from "somewhere else" come in, and do the lion share of melt. Of course melt can't be defined by a graph which defines freeze - melt is much, much more complex thing, since it's not a local process.