It could simply be down to the fact that they're not suited for the 308 and therefore, they're putting the minimum amount they're getting away with on the 308 and using the rest for the Quorum Expresses, X10, X11 and X13 due to the fact that the timetables are less demanding than what the 308 timetable is. Also, the hybrids do tend to struggle on the slip roads on the Coast Road thus making it a struggle to keep to timetable plus the factor of constantly stopping and starting. The reason that they're suited for the Green Angel for though is because the 21 stops at nearly every single stop making them worthwhile for Go North East because the 21 doesn't require the power and speed as much as Arriva's 308 requires to keep to time.