The vegan bread rolls you get on flights are different which I have never quite understood. I suspect their special meals cover off more than one 'special meal request' so the salad instead of sandwich might also be suitable for gluten free, vegan, low salt, low fat, etc.
They may actually come from a completely different catering facility (e.g. if the pastry was also Kosher, it would have to have been made somewhere different from where the main inflight meals are made).
Having a completely different item also means they probably won't make a mistake (e.g. if they had a cheese croissant and a ham and cheese, and the ham was folded up, they might misidentify it).
They also load food pretty tightly (they tend not to have 'leftovers' unless people decline a meal, so if they load 150 croissants for 150 passengers, no need for back ups).
There are also times when the snack offering is veggie (but not vegan, salt free, etc etc) like pizza slices, I read the box and even the cheese was veggie, so I asked if I could have one if there was a spare rather than my sad grilled veggie wrap with no flavor!