Originally Posted by RomeoHotel

If the senior jumpseater for example was an 87' hire date he/she might be way up the list for a seat in the cabin. Meanwhile the junior jumpseater is say a 15' hire date. Way down the list for a cabin seat. The normal etiquette is for the senior guy to stand by for a seat in the cabin to allow the junior jumpseater an opportunity to occupy the jumpseat.
Getting the most senior non rev on in your example would often not be the junior pilot but someone else.