Usually order aircraft announcements are further detailed into "firm orders" and "options". Firm orders kick-off the supply chain long-lead items and configuration, while options only reserve production slots. The Air India Paris Air Show announcement just finalizes a previously announced order from back in February of this year. As Xerxes says, orders get cancelled, slots get assigned to other carriers, and delivered jets get sold if they are excess to capacity needs. I used to work in aircraft design and we would be shown charts of production slots with orders at weekly meetings that constantly changed.
