THYROGLOBULIN (19S) comes to a heavier position than ferritin (51S). Why?
Ferritin and iron-containing ferritin are different and the latter
would of course be heavier; it will therefore sediment faster.
Figure 3 of the third article uses thyroglobulin and ferritin as molecular weight markers. For your information thyroglobulin is 650kDa whereas ferritin is 440kDa. Also note that apoferritin i.e. ferritin that is not bound with iron has a sedimentation rate of 18S (from the same article that measures the sedimentation rate of iron bound ferritin - ref#2).


There seems to be some confusion regarding the sedimentation rate of ferritin. From this article:


  One molecule of ferritin can bind up to 4500 Fe2+ ions.
  When fully loaded with iron, the ferritin can increase in mass up to
  ca 900 kDa; it then has a sedimentation coefficient of 24S (Chasteen &amp;
  Harrison 1999).


55S for ferritin seems dubious.