No, no. You're getting it all wrong. You see, graph presented is about a case when there is no insolation to talk about (at the pole, this starts as early as ~20th September). In such conditions, after several dozens hours, the air naturally cools down to much below 0C (as it radiates some of its energy into space, and there is no Sun to warm it up in a daily 24h cycle). In such conditions, there is a big difference between air temperature (which is much below freezing point) and directly-at-the-surface water (which, because of its huge thermal capacity and convectional mixing cools down dozens, if not hundreds, times slower - and sometimes remain liquid even at surface for quite a while after the Sun goes below the horizon for the polar night). The surface of the ocean is rarely exactly plain as long as it's liquid water - because of winds, there are nearly always some waves, quite often big ones. Mechanical motion creates mixing, and for a while, this prevents ice layer from forming. Not for long though, and depends on conditions. Low/no wind and/or snowing eventually happen, and this is the start of ice formation. Once even very thin layer of ice has formed, THEN you start to see what the graph is about: very rapid growth of ice initially, provided very cold athmosphere (below -20C) remains (which is usually the case).