To do with the 802 having more of its weight on powered axles?The 802's reach 119mph with appx 8.5hp/t - so an 810 with close to 11hp/t should do more than that.
What is interesting here is how HST's and Class 802's with similar power to weight ratios differ in performance. The 802's start off the line much faster in diesel mode, but peter out more quickly - reaching around 119mph on level track - whereas HST's still motor on to 125mph and beyond. Back in the early days - HST speeds around 133 to 136mph were commonly recorded before speed limiters were fitted.
I wonder whether the Class 800/802's seemingly inferior 'ploughed field' roof aerodynamics are a factor here?