BB6 was 'unpredictable' according to Eatock
"NH. Do the variations in the design bear any relation to the persona Big Brother adopts in each series of the programme?
DE. Yes, the designs suggest and reflect the editorial direction that each series takes. For example, when Big Brother was ‘evil’ the eye had spikes, when Big Brother became ‘unpredictable’, the logo was designed to resemble lenticular glass, like that glass that is sometimes used for the front door of someone’s house. They are quite literal and always use a simple shape, lines, circles, triangles, spirals etc."
http://eatock.com/project/big-brother-2/2/