I suspect "atheists", "agnostics" and "non-religious people" describe people that have roughly the same beliefs about the world (in terms of expectations as to what physical things will happen), but have different signalling; atheist signalling that they are affiliated to scientists and hostile to religion; agnostics signalling that they are wise and cautious and not hostile to anybody, "non-religious" just not signalling affiliation to any group based on religious opinion.

Arguably, the same could be said of many moderate and educated religious people: they have the same actual expectations about the world, but choose to affiliate with a religious group for mostly social reasons.