A century consisted of 10 Contubernium (a tent party). It seems a roman tent was actually only large enough for to sleep 8 men - hence the eighty. It may be that originally there were 10 men in a Contubernium - eight in the tent and 2 on watch. see:
http://webpages.charter.net/brueggeman/march-legio
n.html
Centuries in reality contained anything from 30 to 100 plus men, so it seems to make sense that the 80 figure which is usually quoted, has been derived assuming 8 men per Contubernium.