The reason is that the extent covers areas with both very thin ice as well as areas with thick ice. To get a true picture of the state of the sea ice you should assess its volume, and here you have to take the ice thickness into account.

Please remove spaces between words.
Thereasonisthattheextentcoversareaswithbothverythiniceaswellasareaswiththickice.Togetatruepictureofthestateoftheseaiceyoushouldassessitsvolume,andhereyouhavetotaketheicethicknessintoaccount.