They don't serve a different sandwich depending on whether they are flying to or from AMS; if your experience has been that you get the "spread" on the way out and cheese on the way back, that's just coincidence and you've inferred a pattern that doesn't actually exist.

And these sandwiches are only served on a small subset of routes - not on longhaul, obviously, and not on all short routes - and only at certain times of the say. You might also only get a small packet of potato crisps/chips (though I have not seen these in a while) or some other small savoury snack rather than the sandwich dependent on the time of the flight.