Because while England have been on an outstanding run of form over the past 18 months (won 5 out of their last 6 series, and are on course to make that 6/7, with a record of P23 W14 D5 L4), England have been beating mediocre rubbish for that run.
Whereas if you look at the two years prior to that, England lost series to India home and away, South Africa at home, Sri Lanka away and West Indies away, beating only New Zealand home and away and the West Indies at home. Meanwhile, South Africa were winning series in Pakistan, England, Bangladesh and Australia, as well as taking care of New Zealand, Bangladesh and West Indies at home. The only series they failed to win between 2007/08 and losing to Australia at home was their series in India.