I can't remember exactly the explanation I heard, but the gist of it is something happens to the blood that makes it so it will not carry the oxygen properly. So even supplemental oxygen is only partially helpful. Has to do with the platelets being tied up in clotting so not available to carry the oxygen or something along those lines. Basically a low platelet count problem (the best I remember). So the blood problem has to be solved to cure the low O2 level. Supplying more O2 doesn't do a tremendous amount of good if the blood can't carry it.