The main difference between responsibility and accountability is that responsibility can be shared while accountability cannot. Being accountable means being not only responsible for something but also ultimately answerable for your actions.
Also, accountability is something you hold a person to only after a task is done or not done. Responsibility can be before and/or after the task.
It is Tom's responsibility to make sure there are supplies in the office room. So Tom will be aware of this task and keep beinging in more supplies before they run out. At this point, you cannot say it is Tom's accountability. Tom is responsible for the office supplies, but he is only accountable and owes an explanation if the supplies ever run out.