I don't think that thats how it works at the TT. It is the case at world championship level and in certain US race series and possibly on NZ too but at the TT it has just never been the case that last years winners get to ride at number one.
Joey won 26 TTs and never rode at number one
In fact normally the guy at number one is at a disadvantage as he is the target all the rest are trying to catch.