What about the errors each of the contenders have made? That, perhaps more than anything else, has closed things up and made this such a tight finish to the WDC. Red Bull have (according to @EliGP) led 59.3% of all laps this year. Only Ferrari have bettered that this decade, in 2002 & 2004. Given the level of dominance shown by Ferrari in those years Red Bull really should have the WDC wrapped up by now.