I think it's just an experiment to see what people are prepared to do, I would think that the peak time is 4pm until 7pm & they are trying to come up with a way to encourage people to consider their usage between those hours. If they can reduce the peak, then presumably they can refrain from building the network up further, or at least in the short term.
However the cynic may say they are also experimenting as to when they put the high rates in to curb demand via Smart meters
but we all one one fixed rate at the moment via the government unit rate cap ( if on standard flexible plan)......and as we are contracted for that and not for low peak and high peak tariffs that wont change......unless your on their agile plan