How do reality checks work?

The strangest things can seem normal in a dream, so how can something so small as noticing two numbers being differnt trigger lucidity? Talking to a green horse or something in your dream should trigger it as well, but it doesnt seem to.

I am not doubting that they work, I am just asking what you think makes them work?

I have also had dreams where I realise I am dreaming because that is the dream (no reality check) but I do not become aware, the dream just goes on.

How do reality checks effect you differently? I haven't had a reality check in a dream yet so I am in the dark about the whole matter...