Here is an example of a cultural assumption: in the UK, if someone invites you to their house, you don't expect food unless they have specifically invited you for a meal. You would expect an offer of tea and coffee. If the host offered nothing, that would be a little rude. These are cultural assumptions because the guest and the host have the same culture, and the host does not need to say why they are not offering food or why they are offering tea and coffee.