In the wild, a subordinate cat will often bury its poo in order to hide its presence from the dominant cats in the area. An alpha cat may leave their poo uncovered because they are confident of their dominance and wish to assert ownership of their territory.
Domestic cats also display this behaviour. In a household with only one cat, for example, the cat may well bury his poo because he feels that his human companion is dominant. If there is more than one cat in the household, there may be an obviously dominant cat and an obviously subordinate cat, or all cats may consider themselves to be dominant or subordinate!