What also can happen is that the first night is only available with a (more expensive) refundable fare whereas the following nights are also available at a (less expensive) non refundable fare and Accor website can't combine in the same reservation refundable and non refundable fares. In that case it comes cheaper to make separate bookings and ask the reception at check-in to combine them in case they didn't notice you made multiple reservations.
It happened to me when booking hotels in Bangkok : if I book less than 3 days before the stay (more exactly after 1am 2 days before the stay), the first night is only available with a more expensive refundable fare and I save money splitting my reservation in 2 : 1 refundable night and X non refundable nights.