Most food/catering/ and other bulk food prep establishments use this. This is to reduce wastage - in terms of cost, ingredients, labour among other things. I know someone from a meal prep service that caters to a few airlines and they are often told to make sure that the meals are planned in a such a way that - most pax can have that meal and it will satisfy most of the *minimum* dietary requirements.
So I understand that anything *vegetarian* on a flight is going to be the same or similar meal (with one item added/removed) to the baseline vego meal.
Sometimes, the side-veg component of a meat meal will be served as the main-veg component of a vego meal.
The area where most airlines skimp on special meals is the dessert bit. This get a bit tricky from what I'm told. Apparantly, desserts don't hold/freeze/stay fresh for longer than other savory meals, so there is going to be a compromise when it comes to having given a choice. Which is why QF, for a long time, use to give me an apple or an orange or a banana AND dried fruits in a packet AND sultanas along with my special meal. All this comes roped bag - to avoid contaminants.
But I have been able to ask nicely and get an ice cream or other desserts on offer - most of which are not *strictly* vego - as they have eggs in the dessert.