Yea, but i think there's a chicken-egg issue here too. People [low income/whatever] phone in off-peak times -because- it is cheaper, not necessarily because it is a good time of day to phone people. The whole concept of off-peak is kinda artificially created, which now might disappear if the costs are so close between peak/off-peak...
If off-peak and peak had the exact same pricing, would those ratios they throw out there still stand? Maybe some people are now in a routine of phoning everyone at 9pm, but maybe they would equally made the same call at 5pm if it cost the same...
Even with the MTN vs Vodacom ratios being quite different, it might be related to their offerings "forcing" aka "encouraging" people to phone in off-peak times more/less, not the userbase that mysteriously like to phone late at night.