C IoM you say
2 in almost all other time trials they go off in numerical order.

I think , but stand to be corrected , but the TT is unique and I do not know of, or have heard of any other event run in this way , the only other time trials I know of , are trials of endurance where you are given a task to do in a given time slot and are issued with penalties for infringements, usually in the context of added time to achived overall time , bearing no resemblance to the TT so therefore of no importance what number people set off in , more important that the numbers in the program are the numbers on the bikes , which was not the case in the old system in race week ,