I forget the source of this, but I read that it was by design. It was understood that inconsistent censorship and oppression was actually more effective than the consistent type.

Knowing exactly what you are not allowed to say can be comforting: You trust in the rules and there is no reason to be afraid.

Add in inconsistencies and suddenly this comfort is removed. It adds an extra layer of uncertainty and fear to everything. Two people can say the same thing and only one disappears. No one knows where the line lies. It's a nasty psychological game and quite sick and twisted actually.