Because street often only has one or two ways of attacking the obstacle, while at natural, there's a million ways, and as I understand, Ben finds the good lines, making really difficult stuff (for normal people) really easy, just 'cos he finds a good line.
Street is more of the 'throw yourself off a house to a rail 15ft away' and less of the 'cramped runup to muddy rock' variety...
Natural > Street.
Nice video.