A bit of a shot in the dark here but I note that the 231 is the only low frequency route of the four possibilities, the others all being high frequency *every 12 minutes or better) during the day.
In Journey Planner, high frequency bus services have the headway added to the expected journey time. If I make a simialr request of Journey Planner, the journey time is shown as 8-9 minutes when a 231 is the option shown and more like 20-21 minutes when it is not.
This is not made clear to the user, nor is there any real logic to it (tube services do not get similar treatment). It is quite feasible that a 231 will be shown as the best option on Journey Planner even where ti isn’t.
Maybe that does not matter when all services serve the same stop anyway but there are examples where the Journey Planner approach would send you to the wrong stop. Three routes run from Enfield Town to Chase Farm Hospital, with no common stop. JP will probably advise you to go the 313 or W9 stop even though the high frequency W8 is often the better option.
If there is any link between what JP does and what your call does, I would expect to see 217 231 and 444 cropping up regularly for your call if made in the evenings but the 144 not so often, as it is the only one of the four that is still high frequency.