When switching a meal
The app tries to find meals which, after switching, will ensure that the overall calorie target for the day still sits within the overall target for you. This is why, if you have two snacks which have different calorie amounts, you may see different switch options for both - the overall goal of the app is to present meals that, after you switch them, will ensure you still sit within your target.

On the recipe tab
The app has no context of a meal being switched. It therefore picks a calorie target and finds all the meals which can be scaled to that target. So you will see a larger selection of meals in the whole recipe library, but may not be presented with them on certain days.